CHAPTER VII.OF THE CIRCULATION.
Vessels connected with the heart: chambers of the heart—Position of the heart—Pulmonic circle: systemic circle—Structure of the heart, artery, and vein—Consequences of the discovery of the circulation to the discoverer—Action of the heart: sounds occasioned by its different movements—Contraction: dilatation—Disposition and action of the valves—Powers that move the blood—Force of the heart—Action of the arterial tubes: the pulse: action of the capillaries: action of the veins—Self-moving power of the blood—Vital endowment of the capillaries: functions—Practical applications.
Vessels connected with the heart: chambers of the heart—Position of the heart—Pulmonic circle: systemic circle—Structure of the heart, artery, and vein—Consequences of the discovery of the circulation to the discoverer—Action of the heart: sounds occasioned by its different movements—Contraction: dilatation—Disposition and action of the valves—Powers that move the blood—Force of the heart—Action of the arterial tubes: the pulse: action of the capillaries: action of the veins—Self-moving power of the blood—Vital endowment of the capillaries: functions—Practical applications.
252. The blood, being necessary to nourish the tissues and to stimulate the organs, must be in motion in order to be borne to them. An apparatus is provided partly for the purpose of originating an impelling force to put the blood in motion, and partly for the purpose of conveying the blood when in motion to the different parts of the body.
253. The heart is the impelling organ; the great vessels in immediate connexion with it are the transmitting organs (fig. CXIV. 1, 2). The heart is divided into two sets of chambers (fig. CXIV. 3, 4, 10, 11), one for the reception of the blood from the different parts of the body (fig. CXIV. 3, 10); the other for the communication of theimpulse which keeps the blood in motion (fig. CXIV. 4, 11). The chamber which receives the blood is termed an auricle (fig. CXIV. 3, 10), and is connected with a vessel termed a vein (fig. CXIV. 1, 2, 9); that which communicates impulse to the blood is termed a ventricle (fig. CXIV. 4, 11), and is connected with a vessel termed an artery (fig. CXIV. 7, 12). The vein carries blood to the auricle; the auricle transmits it to the ventricle; the ventricle propels it into the artery; the artery, carrying it out from the ventricle, ultimately sendsit again into the vein, the vein returns it to the auricle, the auricle to the ventricle, the ventricle to the artery, and thus the blood is constantly moving in a circle; hence the name of the process, the circulation of the blood.
Fig. CXIV.View of the heart with its several chambers exposed, andthe great vessels in connection with them. 1. The superiorvena cava; 2. the inferior vena cava; 3. the chamber calledthe right auricle; 4. the chamber called the right ventricle;5. the line marking the passage between the two chambers,and the points of attachment of one margin of the valve;6. the septum between the two ventricles; 7. the pulmonaryartery arising from the right ventricle, and dividing at 8, intoright and left for the corresponding lungs; 9. the four pulmonaryveins bringing the blood from the lungs into 10,the left auricle; 11. the left ventricle; 12. the aorta arisingfrom the left ventricle, and passing down behind the heartto distribute blood, by its divisions and subdivisions, toevery part of the body.
View of the heart with its several chambers exposed, andthe great vessels in connection with them. 1. The superiorvena cava; 2. the inferior vena cava; 3. the chamber calledthe right auricle; 4. the chamber called the right ventricle;5. the line marking the passage between the two chambers,and the points of attachment of one margin of the valve;6. the septum between the two ventricles; 7. the pulmonaryartery arising from the right ventricle, and dividing at 8, intoright and left for the corresponding lungs; 9. the four pulmonaryveins bringing the blood from the lungs into 10,the left auricle; 11. the left ventricle; 12. the aorta arisingfrom the left ventricle, and passing down behind the heartto distribute blood, by its divisions and subdivisions, toevery part of the body.
254. In nourishing the tissues and stimulating the organs, the blood parts with its nutritive and stimulating constituents, and receives in return some ingredients which can no longer be usefully employed in the economy, and others which are positively injurious. An apparatus is established for its renovation and depuration; this organ is termed the lung (fig. LIX. 5), and to this organ the blood must in like manner be conveyed. Thus the blood moves in a double circle, one from the heart to the body and from the body back to the heart, termed the systemic circle; the other from the heart to the lung and from the lung back to the heart, termed the pulmonic circle. Hence in the human body the heart is double, consisting of two corresponding parts precisely the same in name, in nature, and in office; the one appropriated to the greater, or the systemic, and the other to the lesser, or the pulmonic circulation (fig. CXIV.).
255. There is a complete separation between these two portions of the heart (fig. CXIV. 6), formed by a strong muscular partition which prevents any communication between them except through the medium of vessels.
256. The heart is situated between the two lungs (fig. LIX. 2, 5), in the lower and fore part ofthe chest, nearly in the centre, but inclining a little to the left side. Its position is oblique (fig. LIX. 2, 5). Its basis is directed upwards, backwards, and towards the right (fig. LIX. 2); its apex is directed downwards, forwards, and towards the left, opposite to the interval between the cartilages of the fifth and sixth ribs (fig. LIX. 2). It is inclosed in a bag termed the pericardium (fig. CXV.), which consists of serous membrane.The pericardium is considerably larger than the heart, allowing abundant space for the action of the organ (fig. CXV.). One part of the pericardium forms a bag around the heart (fig. CXV.); the other part is reflected upon the heart so as to form its external covering (fig. CXV.), and is continued for a considerable distance upon the great vessels that go to and from the heart in such a manner that this bag, like all the serous membranes, constitutes a shut sac. Both that portion of the pericardium which is reflected upon the heart, and that which forms the internal surface of the bag around it, is moistened during life by a serous fluid, which, after death, is condensed into a small quantity of transparent water. That portion of the pericardium which rests on the diaphragm (fig. LXX. 1) is so firmly attached to it that it cannot be separated without laceration, and by this attachment, together with the great vessels at its base, the heart is firmly held in its situation, although in the varied movements of the body it is capable of deviating to a slight extent from the exact position here described.
Fig. CXV.View of the heart enveloped in its pericardium, the fore partof the latter being cut open and reflected back.
View of the heart enveloped in its pericardium, the fore partof the latter being cut open and reflected back.
257. When the interior of the heart is laid open there are brought into view four chambers (fig. CXIV. 3, 4, 10, 11), two for each circle. Those belonging to the pulmonic circle are on the right (fig. CXIV. 3, 4), those to the systemic on the left side of the body (fig. CXIV. 10, 11);hence the terms right and left are applied to these respective parts of the heart.
258. The veins which carry the blood to the right or the pulmonic chambers are two, one of which brings it from the upper, and the other from the lower parts of the body: the first is called the superior and the second the inferior vena cava (fig. CXIV. 1, 2). Both pour their blood into the first chamber, termed the right auricle (fig. CXIV. 3); from the right auricle the blood passes into the second chamber, denominated the right ventricle (fig. CXIV. 4): from which springs the artery which carries the blood from the heart to the lung, the pulmonary artery (fig. CXIV. 7). This is the pulmonic circle. From the lung the blood is returned to the heart by four veins, termed the pulmonary veins (fig. CXIV. 9), which pour the blood into the third chamber of the heart, the left auricle (fig. CXIV. 10). From the left auricle it passes into the fourth chamber, the left ventricle (fig. CXIV. 11), from which springs the artery which carries out the blood to the system, termed the aorta (figs. CXIV. 12, and CXVII. 11). This is the systemic circle. In the system the minute branches of the aorta unite with the minute branches that form the venæ cavæ, which return the blood to the right auricle of the heart, and thus the double circle is completed.
259. The two chambers called the auriclesoccupy the basis of the heart (fig. CXIV. 3, 10). The right auricle is situated at the basis of the right ventricle (figs. CXIV. 3, and CXVI. 4). Itis partly membranous and partly muscular. At its upper and back part is the opening of the vena cava superior (fig. CXVI. 1), which returns the blood to the heart from the head, neck, and all the upper parts of the body. At its lower part is the opening of the vena cava inferior (fig. CXVI. 2), which returns the blood from all the lower parts of the body.
Fig. CXVI.View of the heart with the great vessels in connectionwith it, on the right side, its different chambers being laidopen and its structure shown. 1. The vena cava superior;2. the vena cava inferior; 3. cut edge of the right auricleturned aside to show, 4. the cavity of the right auricle intowhich the two venæ cavæ pour the blood returned from allparts of the body; 5. hook suspending the reflected portionof the wall of the auricle; 6. the right ventricle; 7. cutedge of the wall of the ventricle, a portion of which hasbeen removed to show 8. the cavity of the ventricle; 9.situation of the opening between the auricle and ventricle,called the auricular orifice of the ventricle; 10. valve placedbetween the auricle and ventricle, one margin being firmlyattached to the auriculo-ventricular opening in its entireextent, the other lying loose in the cavity of the ventricle;11. probe passed from the auricle into the ventricle underneaththe valve, showing the course of the blood from theformer chamber to the latter; 12. the columnæ carneæattached by one extremity to the walls of the ventricle, theother extremity ending in tendinous threads attached to theloose margin of the valve; 13. passage to the pulmonaryartery; 14. the three semilunar valves placed at the commencementof 15. the pulmonary artery; 16. the two greatbranches into which the trunk of the pulmonary arterydivides, one branch going to each lung.
View of the heart with the great vessels in connectionwith it, on the right side, its different chambers being laidopen and its structure shown. 1. The vena cava superior;2. the vena cava inferior; 3. cut edge of the right auricleturned aside to show, 4. the cavity of the right auricle intowhich the two venæ cavæ pour the blood returned from allparts of the body; 5. hook suspending the reflected portionof the wall of the auricle; 6. the right ventricle; 7. cutedge of the wall of the ventricle, a portion of which hasbeen removed to show 8. the cavity of the ventricle; 9.situation of the opening between the auricle and ventricle,called the auricular orifice of the ventricle; 10. valve placedbetween the auricle and ventricle, one margin being firmlyattached to the auriculo-ventricular opening in its entireextent, the other lying loose in the cavity of the ventricle;11. probe passed from the auricle into the ventricle underneaththe valve, showing the course of the blood from theformer chamber to the latter; 12. the columnæ carneæattached by one extremity to the walls of the ventricle, theother extremity ending in tendinous threads attached to theloose margin of the valve; 13. passage to the pulmonaryartery; 14. the three semilunar valves placed at the commencementof 15. the pulmonary artery; 16. the two greatbranches into which the trunk of the pulmonary arterydivides, one branch going to each lung.
260. The auricle communicates with its corresponding ventricle by a large opening, termed the auricular orifice of the ventricle (figs. CXIV. 5, and CXVI. 9). All around the opening is placed a thin but strong membrane (fig. CXVI. 10), one margin of which is firmly attached to the wall of the ventricle (figs. CXIV. 5, and CXVI. 9), while the other is free (fig. CXVI. 10). This membrane receives the name, and, as will be seen immediately, performs the office of a valve.
261. The ventricle is much thicker and proportionally stronger than the auricle (fig. CXVI. 3, 6). It is composed almost entirely of muscular fibre. Over nearly the whole extent of its internal surface are placed irregular masses of muscular fibres, many of which stand out from the wall of the ventricle like columns or pillars (fig. CXVI. 12); hence they are called fleshy columns (columnæ carneæ). Some of these fleshy columns are adherent by one extremity to the wall of the ventricle, while the other extremity terminates in tendinous threads which are attached to the membrane that forms the valve (fig. CXVI. 12).
262. From the upper and right side of this chamber springs the pulmonary artery (fig. CXVI. 15); at the entrance of which are placed three membranes of a crescent or semilunar shape, termed the semilunar valves (fig. CXVI. 14).
263. The structure of the left side of the heart is perfectly analogous to that of the right. Its auricle, like that on the left side, is placed at the base of the ventricle (figs. CXIV. 10, and CXVII. 2), and like it also is thin, being composed chiefly of membrane. At its upper and back part (figs. CXIV. 9, and CXVII. 1) are the openings of the four pulmonary veins, two from the right, and two from the left lung.
264. At the passage of communication between the left auricle and ventricle is placed a valve analogous to that on the right side (fig. CXVII. 7).
Fig. CXVII.View of the heart with the great vessels in connectionwith it, on the left side, its chambers being laid open as inthe preceding figure. 1. The four pulmonary veins openinginto, 2. the cavity of the left auricle; 3. the cut edge of thewall of the auricle; 4. the appendix of the auricle; 5. thecavity of the left ventricle; 6. the cut edge of the wall ofthe ventricle, the greater portion of the wall having beenremoved to show the interior of the chamber; 7. valveplaced between the auricle and ventricle; 8. columnæ carneæterminating in tendinous threads attached to the loosemargin of the valve; 9. probe passed underneath the valveand its tendinous threads, raising them from the wall ofthe ventricle similar to a refluent current of blood; 10.passage to 11. the aorta; 12. two of the semilunar valvesplaced at the mouth of the aorta, the third having been cutaway; 13. arch of the aorta; 14. the three semilunar valvesat the commencement of the pulmonary artery seen inaction, completely closing the mouth of the vessel.
View of the heart with the great vessels in connectionwith it, on the left side, its chambers being laid open as inthe preceding figure. 1. The four pulmonary veins openinginto, 2. the cavity of the left auricle; 3. the cut edge of thewall of the auricle; 4. the appendix of the auricle; 5. thecavity of the left ventricle; 6. the cut edge of the wall ofthe ventricle, the greater portion of the wall having beenremoved to show the interior of the chamber; 7. valveplaced between the auricle and ventricle; 8. columnæ carneæterminating in tendinous threads attached to the loosemargin of the valve; 9. probe passed underneath the valveand its tendinous threads, raising them from the wall ofthe ventricle similar to a refluent current of blood; 10.passage to 11. the aorta; 12. two of the semilunar valvesplaced at the mouth of the aorta, the third having been cutaway; 13. arch of the aorta; 14. the three semilunar valvesat the commencement of the pulmonary artery seen inaction, completely closing the mouth of the vessel.
265. The walls of the left ventricle are nearly as thick again as those of the right, and its fleshy columns are much larger and stronger. From the upper and back part of this fourth chamber (fig. CXVII. 11) springs the great systemic artery, the aorta, around the mouth of which are placed three semilunar valves (fig. CXVII. 12), similar to those at the mouth of the pulmonary artery.
266. The partition which divides the two sets of chambers from each other (fig. CXIV. 6) is wholly composed of muscular fibres, and is called the septum of the heart.
267. The external surface of the heart is covered by a thin but strong membrane continued over it from the pericardium. Between this membranous covering and its fleshy substance is lodged, even when the body is reduced to the greatest degree of thinness, a quantity of fat. Immediately beneath this fat are the fleshy fibres that compose the main bulk of the organ. These fibres are arranged in a peculiar manner. The arrangement is not perceptible when the heart is examined in its natural state, but after it has been subjected to long-continued boiling, which, besides separating extraneous matters from the fibres, hardens and loosens without displacing them, the manner in which they are disposed is manifest. Just at the point where the muscular fibres that constitute the septum of the auricles are set upon those which form the septum of the ventricles, and parallel with the origin of theaorta, the heart is not muscular but tendinous. The substance called tendon, it has been shown, is often employed in the body to afford origin or insertion to muscular fibres, performing, in fact, the ordinary office of bone, and substituted for it in situations where bone would be inconvenient. From the tendinous matter just indicated most of the fibres that constitute the muscular walls of the heart take their origin. From this point the fibres proceed in different directions: those which go to form the wall of the auricles ascend; those which form the wall of the ventricles pursue an oblique course downwards, and the arrangement of the whole is such, that a general contraction of the fibres must necessarily bring all the parts of the heart towards this central tendinous point. The object and the result of this arrangement will be manifest immediately.
268. The internal surface of the chambers of the heart, in its whole extent, is lined by a fine transparent serous membrane, which renders it smooth and moist; and, like all other organs which have important functions to perform, it is plentifully supplied with blood-vessels and nerves.
269. Such is the structure of the organ that moves the blood. The artery, the tube that carries it out from the heart, is a vessel composed of three distinct layers of membrane superimposed one upon another, and intimately united by delicate cellular tissue. These layers are termedtunics or coats. The external coat (fig. CXVIII. 3), which is also called the cellular, consists of minute whitish fibres, which are dense and tough, and closely interlaced together in every direction. They form a membrane of great strength, the elasticity of which, especially in the longitudinal direction, is such that, in addition to its other names, it has received that of the elastic coat.
Fig. CXVIII.Portion of an artery, showing the several coats of whichit is composed separated from each other. 1. The internalor serous coat; 2. the middle or fibrous coat; 3. the externalor cellular coat.
Portion of an artery, showing the several coats of whichit is composed separated from each other. 1. The internalor serous coat; 2. the middle or fibrous coat; 3. the externalor cellular coat.
270. The middle or the fibrous tunic is composed of yellowish flattened fibres which pass in an oblique direction around the calibre of the vessel, forming segments of circles, which, uniting, produce complete rings (fig. CXVIII. 2). This tunic is thick, consisting of several layers of fibres which it is easy to peel off in succession. They form a firm, solid, elastic, but, at the same time, brittle membrane.
271. The inner tunic, thin, colourless, nearly transparent, and perfectly smooth, is moistened by a serous fluid, and is thence called the serous coat (fig. CXVIII. 1). To the naked eye it presents no appearance of fibres, yet notwithstanding its extreme delicacy, it is so strong that, after the other coats of the artery have been entirely removed in a living animal, it is capable of resisting the impetus of the circulation, and of preventing the dilatation of the artery. The arteries themselves are supplied with arteries, vessels that nourish their tissues, and which are sent to them from neighbouring branches, seldom or never from the vessel itself to which they are distributed. Each individual part of an artery is supplied by its own appropriate vessels, which form but few communications above and below, so that if care be not taken in surgical operations to disturb these nutrient arteries very little, the vessel will perish for want of sustenance.
272. The vein, the tube that carries back the blood to the heart, is composed of the same number of tunics as the artery, which, with the exception of the middle, are essentially the same in structure, but they are all much thinner. The external tunic consists of a less dense and strong cellular membrane; the middle tunic, instead of being formed of elastic rings, is composed of soft and yielding fibres, disposed in a longitudinal direction; while the inner coat, which is still more delicatethan that of the artery, is arranged in a peculiar manner. The inner coat of most veins, at slight intervals, is formed into folds (fig. CXX. 5), one margin of which is firmly adherent to the circumference of the vessel, while the other margin is free and turned in the direction of the heart. These membranous folds are termed valves. In all veins the diameter of which is less than a line the valves are single; in most veins of greater magnitude they are placed in pairs, while in some of the larger trunks they are triple, and in a few instances quadruple, and even quintuple. The veins, like the arteries, are supplied with nutrient vessels and nerves.
273. All the arteries of the body proceed from the two trunks already described; that connected with the pulmonic circle, the pulmonary artery, and that connected with the systemic circle, the aorta. These vessels, as they go out from the heart and proceed to their ultimate termination, are arborescent, that is, they successively increase in number and diminish in size, like the branches of a tree going off from the trunk (fig. CXIX. 1, 2, 3). Each trunk usually ends by dividing into two or more branches (fig. CXIX. 1, 2), the combined area of which is always greater than that of the trunk from which they spring, in the proportion of about one and a half to one. As the branch proceeds to its ultimate termination it divides and subdivides, until at length the vessel becomes sominute, that it can no longer be distinguished by the eye. These ultimate branches are called capillary vessels, from their hair-like smallness (fig. CXIX. 4); but this term does not adequately express their minuteness. It has been stated (234) that the red particle of the blood, at the medium calculation, is not more than the three-thousandth part of an inch in diameter; yet vast numbers of the capillary vessels are so small that they are incapable of admitting one of these particles, and receive only the colourless portion of the blood.
Fig. CXIX.View of the manner in which an artery divides and subdividesinto its ultimate branches. 1. Trunk of the artery;2. large branches into which it subdivides; 3. smallbranches, successively becoming smaller and smaller untilthey terminate in 4. the capillary branches.
View of the manner in which an artery divides and subdividesinto its ultimate branches. 1. Trunk of the artery;2. large branches into which it subdivides; 3. smallbranches, successively becoming smaller and smaller untilthey terminate in 4. the capillary branches.
274. Every portion of an artery, by reason of the elasticity of its coats, preserves nearly a cylindrical form, and as the area of the branches is greater than that of the trunks, the blood, in proceeding from the heart to the capillaries, though passing through a series of descending cylinders, is really flowing through an enlarging space.
275. The disposition of the veins, like that of the arteries, is arborescent, but in an inverse order; for the course of the veins is from capillary vessels to visible branches, and from visible branches to large trunks (fig. CXX. 1, 2, 3). In every part of the body where the capillary arteries terminate the capillary veins begin, and the branches uniting to form trunks, and the small to form large trunks, and the trunks always advancing towards the heart, and always increasing in magnitude as they approach it, form at length the two veins which ithas been stated (258) return all the blood of the body to the right auricle of the heart.
Fig. CXX.View of the manner in which the minute branches of thevein unite to form the larger branches and the trunks.1. Capillary venous branches; 2. small branches formed bythe union of the capillary; 3. larger branches formed bythe union of the smaller and gradually increasing in size,to form the great trunk, 4. a portion of which is laid opento show its inner surface and the arrangement of 5. thevalves formed by its inner coat.
View of the manner in which the minute branches of thevein unite to form the larger branches and the trunks.1. Capillary venous branches; 2. small branches formed bythe union of the capillary; 3. larger branches formed bythe union of the smaller and gradually increasing in size,to form the great trunk, 4. a portion of which is laid opento show its inner surface and the arrangement of 5. thevalves formed by its inner coat.
276. The veins are very much more numerous than the arteries, for they often consist of double sets, and they are at the same time more capacious and more extensible. Reckoning the whole of the blood at one-fifth of the weight of the body, it is estimated that, of this quantity, about one-fourth is in the arterial and the remaining three-fourths in the venous system. The combined area of the branches of the veins is much greater than that of the two trunks in which they terminate (fig. CXX. 1, 2, 3, 4): the blood, therefore, in returning to the heart, is always flowing from a large into a smaller space.
277. The divisions and subdivisions of the artery freely communicate in all parts of the body by means of what are called anastomosing branches, and this communication of branch with branch and trunk with trunk is termed anastomosis. The same intercommunication, but with still greater freedom and frequency, takes place among the branches of veins. In both orders of vessels the communication is frequent in proportion to the minuteness of the branch and its distance fromthe heart. It is also more frequent in proportion as a part is exposed to pressure; hence the minute arteries and veins about a joint are distinguished for the multitude of their anastomosing branches; and above all, it is frequent in proportion to the importance of the organ; hence the most remarkable anastomosis in the body is in the brain. By this provision care is taken that no part be deprived of its supply of blood; for if one channel be blocked up, a hundred more are open to the current, and the transmission of it to any particular region or organ by two or more channels, instead of through one trunk, is a part of the same provision. Thus the fore-arm possesses four principal arteries with corresponding veins, and the brain receives its blood through four totally independent canals[6].
278. That the blood is really a flowing stream, and that it pursues the course described (258), is indubitable. For,
(1.) With the microscope, in the transparent parts of animals, the blood can be seen in motion (fig. CXXI.); and if its course be attentively observed, its route may be clearly traced.
Fig. CXXI.View of the circulation of the blood as seen under themicroscope in the web of the frog's foot.
View of the circulation of the blood as seen under themicroscope in the web of the frog's foot.
(2.) The membranes termed valves are so placed as to allow of the freest passage to the blood in the circle described, while they either altogether prevent or exceedingly impede its movement in any other direction.
(3.) The effect of a ligature placed around a vein and an artery, and of a puncture made above the ligature in the one vessel and below it in the other, demonstrate both the motion of the blood and the course of it. When a ligature is placed around a vein, that part of the vessel which is most distant from the heart becomes full and turgid on account of the accumulation of blood in it; while the part of the vessel which is between the ligature and the heart becomes empty andflaccid, because it has carried on its contents to the heart, and it can receive no fresh supply from the body. When, on the contrary, a ligature is placed around an artery, that portion of the vessel which lies between the ligature and the heart becomes full and turgid, and the other portionempty and flaccid. This can only be because the contents of the two vessels move in opposite directions,—from the heart to the artery, from the artery to the vein, and from the vein to the heart. At the same time, if the vein be punctured above the ligature, there will be little or no loss of blood; while if it be punctured below the ligature, the blood will continue to flow until the loss of it occasions death, which could not be unless the blood were in motion, nor unless the direction of its course were from the artery to the vein and from the vein to the heart.
(4.) If fluids be injected into the veins or arteries, whether of the dead or of the living body, they readily make their way and fill the vessels, if thrown in the direction stated to be the natural course of the circulation; but they are strongly resisted if forced in the opposite direction.
279. Such is the description, and with the exception of the first proof, such the evidence of the circulation of the blood in the human body, pretty much as it was given by the discoverer of it, the illustrious Harvey. Before the time of Harvey, a vague and indistinct conception that the blood was not without motion in the body had been formed by several anatomists. It is analogous to the ordinary mode in which the human mind arrives at discovery (chap. iii., p. 103), that many minds should have an imperfect perception of an unknown truth, before some one mind sees it in its completenessand fully discloses it. Having, about the year 1620, succeeded in completely tracing the circle in which the blood moves, and having at that time collected all the evidence of the fact, with a rare degree of philosophical forbearance, Harvey still spent no less than eight years in re-examining the subject, and in maturing the proof of every point, before he ventured to speak of it in public. The brief tract which at length he published was written with extreme simplicity, clearness, and perspicuity, and has been justly characterised as one of the most admirable examples of a series of arguments deduced from observation and experiment that ever appeared on any subject.
280. Cotemporaries are seldom grateful to discoverers. More than one instance is on record in which a man has injured his fortune and lost his happiness through the elucidation and establishment of a truth which has given him immortality. It may be that there are physical truths yet to be brought to light, to say nothing of new applications of old truths, which, if they could be announced and demonstrated to-day, would be the ruin of the discoverer. It is certain that there are moral truths to be discovered, expounded, and enforced, which, if any man had now penetration enough to see them, and courage enough to express them, would cause him to be regarded by the present generation with horror and detestation. Perhaps, during those eight years of re-examination,the discoverer of the circulation sometimes endeavoured in imagination to trace the effect which the stupendous fact at the knowledge of which he had arrived would have on the progress of his favourite science; and, it may be, the hope and the expectation occasionally arose that the inestimable benefit he was about to confer on his fellow men would secure to him some portion of their esteem and confidence. What must have been his disappointment when he found, after the publication of his tract, that the little practice he had had as a physician, by degrees fell off. He was too speculative, too theoretical, not practical. Such was the view taken even by his friends. His enemies saw in his tract nothing but indications of a presumptuous mind that dared to call in question the revered authority of the ancients; and some of them saw, moreover, indications of a malignant mind, that conceived and defended doctrines which, if not checked, would undermine the very foundations of morality and religion. When the evidence of the truth became irresistible, then these persons suddenly turned round and said, that it was all known before, and that the sole merit of this vaunted discoverer consisted in having circulated the circulation. The pun was not fatal to the future fame of this truly great man, nor even to the gradual though slow return of the public confidence even during his own time; for he lived to attain the summit of reputation.
281. It is then indubitably established that the whole blood of the body in successive streams is collected and concentrated at the heart. The object of the accumulation of a certain mass of it at this organ is to subject it to the action of a strong muscle, and thereby to determine its transmission with adequate force and precision through the different sets of capillary vessels.
282. In the accomplishment of this object the heart performs a twofold action; that of contraction and that of dilatation. The auricles contract and thereby diminish their cavities, then dilate and thereby expand them, and the one action alternates with the other. There is the like alternate contraction and dilatation of the ventricles. The first action is termed systole, the second diastole, and both are performed with force.
283. When the heart is laid open to view in a living animal, and its movements are carefully observed, it is apparent that the two auricles contract together; that the two ventricles contract together; that these motions alternate with each other, and that they proceed in regular succession. The interval between these alternate movements is, however, exceedingly short, and can scarcely be perceived when the heart is acting with full vigour; but it is evident when its action is somewhat languid.
284. When the ventricles contract, the apex of the heart is drawn upwards, and at the same timeraised or tilted forwards. It is during this systole of the ventricles, and in consequence of this result of their action, that the apex of the heart gives that impulse against the walls of the chest which is felt in the natural state between the fifth and sixth ribs, and which just perceptibly precedes the pulse at the wrist.
285. When the ear is applied to the human chest, over the situation of the heart, a dull and somewhat prolonged sound is heard, which precedes and accompanies the impulse of the heart against the chest. This dull sound is immediately succeeded by a shorter and sharper sound: after this there is a short pause; and then the dull sound and impulse are again renewed. The duller sound and stronger impulse are ascribed to the contraction of the ventricles, and the sharper sound and feebler impulse to that of the auricles.
286. The movement of the heart is effected by the contraction of its muscular fibres. Those fibres rest, as upon a firm support, on the tendinous matter to which they are attached, from which they diverge, and towards which their contraction must necessarily bring all the parts of the heart (267). The result of their contraction is the powerful compression of all the chambers of the heart, and thereby the forcible ejection of their contents through the natural openings.
287. But the chambers, alternately with forcible contraction, perform the action of forcible dilatation.This movement of dilatation is effected by the reaction of the elasticity of the tendinous matter on which the muscular fibres are supported (267). This highly elastic substance, by the contraction of the fibres, is brought into a state of extreme tension. The contraction of the fibres ceasing, that moment the tense tendon recoils with a force exactly proportionate to the degree of tension into which it had been brought. Thus the very agent that is employed forcibly to close the chamber is made the main instrument of securing its instantaneous re-opening. A vital energy is appointed to accomplish what is indispensable, and what nothing else can effect, the origination of a motive power; a physical agent is conjoined to perform the easier task to which it is competent; and the two powers, the vital and the physical, work in harmony, each acting alternately, and each, with undeviating regularity and unfailing energy, fulfilling its appropriate office.
288. When the chambers of the heart which open into each other, and which as freely communicate with the great vessels that enter and proceed from them, are forcibly closed, and the blood they contain is projected from them, how is one uniform forward direction given to the current? Why, when the right ventricle contracts, is the blood not sent back into the right auricle, as well as forward into the pulmonary artery? There is but one mode of preventing such an event, which is to place a flood-gatebetween the two chambers; and there a flood-gate is placed, and that flood-gate is the valve. As long as the blood proceeds onwards in the direct course of the circulation, it presses this membrane close to the side of the heart, and thereby prevents it from occasioning any impediment to the current. When, on the contrary, the blood is forced backwards, and attempts to re-enter the auricle, being of course driven in all directions, some of it passes between the wall of the ventricle and the valve. The moment it is in this situation it raises up the valve, carries it over the mouth of the passage, and shuts up the channel. There cannot be a more perfect flood-gate.
289. This is beautiful mechanism; but there is another arrangement which surpasses mere mechanism, however beautiful. It has been shown (260) that one edge of the membrane that forms the valve is firmly adherent to the wall of the ventricle, while the other edge, when not in action, appears to lie loosely in the ventricle (fig. CXVI. 10). Were this edge really loose the refluent current would carry it back completely into the auricle, and so counteract its action as a valve; but it is attached to the tendinous threads proceeding from the fleshy columns that stand along the wall of the ventricle (fig. CXVI. 12). By these tendinous threads, as by so many strings, the membrane is firmly held in its proper position (fig. CXVI. 10, 12); and the refluent currentcannot carry it into the auricle. Thus far the arrangement is mechanical. But each of these fleshy columns is a muscle, exerting a proper muscular action. Among the stimulants which excite the contractility of the muscular fibre, one of the most powerful is distension. The refluent current distends the membrane; the distension of the membrane stretches the tendinous threads attached to it; the stretching of its tendinous threads stretches the fleshy column; by this distension of the column it is excited to contraction; by the contraction of the column its thread is shortened; by the shortening of the thread the valve is tightened, and that in the exact degree in which the thread is shortened. So, the greater the impetus of the refluent blood, the greater the distension of the membrane; and the greater the distension of the membrane, the greater the excitement of the fleshy column; the greater the energy with which it is stimulated to act, the greater, therefore, the security that the valve will be held just in the position that is required, with exactly the force that is needed. Here, then, is a flood-gate not only well constructed as far as regards the mechanical arrangement, but so endowed as to be able to act with additional force whenever additional force is requisite; to put forth on every occasion, as the occasion arises, just the degree of strength required, and no more.
290. The contraction of the heart is the power that moves the blood; and this contraction generatesa force which is adequate to impel it through the circle. From experiments performed by Dr. Hales it appears that if the artery of a large animal, such as the horse, be made to communicate with an upright tube, the blood will ascend in the tube to the height of about ten feet above the level of the heart, and will afterwards continue there rising and falling a few inches with each pulsation of the heart. In this animal, then, the heart acts with a force capable of maintaining a column of ten feet. Now a column of ten feet indicates a pressure of about four pounds and a half in a square inch of surface. Suppose the human heart to be capable of supporting a column of blood eight feet high, this will indicate a pressure of four pounds to the square inch; but the left ventricle of the heart, while it injects its column of blood into the aorta, has to overcome the inertia of the quantity of blood projected; of the mass already in the artery, and of the elasticity of the vessel yielding to a momentary increase of pressure: it is probable, therefore, that the heart acts with a force of six pounds on the inch. The left ventricle, when distended, has about ten square inches of internal surface; consequently the whole force exerted by it may be about sixty pounds. According to the calculation of Hales, it is fifty-one and a half. Now, it is proved by numerous experiments, that, after death, a slight impulse with the syringe, certainly much less than that which is acting upon the blood in the sameartery during life, is sufficient to propel a solution of indigo, or fresh drawn blood, from a large artery into the extreme capillary. If, therefore, after death, a slight force will fill the capillaries, a force during life equal to sixty pounds must be adequate to do so.
291. The heart, with a force equal to the pressure of sixty pounds, propels into the artery two ounces of blood at every contraction. It contracts four thousand times in an hour. There passes through the heart, therefore, every hour, eight thousand ounces or seven hundred pounds of blood. It has been stated (216) that the whole mass of blood in an adult is about twenty-eight pounds: on an average the entire circulation is completed in two minutes and a half; consequently a quantity of blood equal to the whole mass passes through the heart from twenty to twenty-four times in an hour. But though the average space of time requisite to accomplish a complete circulation may be two minutes and a half, yet when a stream of blood leaves the heart, different portions of it must finish their circle at very different periods, depending in part upon the length of the course which they have to go, and in part upon the degree of resistance that obstructs their passage. A part of the stream, it is obvious, finishes its course in circulating through the heart itself; another portion takes a longer circuit through the chest; another extends the circle round the head; andanother visits the part placed at the remotest distance from the central moving power. Such is the velocity with which the current sometimes goes, that, in the horse, a fluid injected into the great vein of the neck, on one side, has been detected in the vein on the opposite side, and even in the vein of the foot, within half a minute.
292. It has been shown (282) that the different chambers of the heart have a tendency to perform their movements in a uniform manner, and in a successive order; that they contract and dilate in regular alternation, and at equal intervals; but, moreover, they continue these movements equally without rest and without fatigue. On go the motions, night and day, for eighty years together, at the rate of a hundred thousand strokes every twenty-four hours, alike without disorder, cessation, or weariness. The muscles of the arm tire after an hour's exertion, are exhausted after a day's labour, and can by no effort be made to work beyond a certain period. There is no appreciable difference between the muscular substance of the heart and that of the arm. It is true that the heart is placed under one condition which is peculiar. Muscles contract on the application of stimuli; and different muscles are obedient to different stimuli,—the voluntary muscles to the stimulus of volition, and the heart to that of the blood. The exertion of volition is not constant, but occasional; the muscle acts only when it is excitedby the application of its stimulus: hence the voluntary muscle has considerable intervals of rest. The blood, on the contrary, is conveyed to the heart without ceasing, in a determinate manner, in a successive order; and this is the reason why through life its action is uniform: it uniformly receives a due supply of its appropriate stimulus. But why it is unwearied, why it never requires rest, we do not know. We know the necessities of the system which render it indispensable that it should be capable of untiring action, for we know that the first hour of its repose would be the last of life; but of the mode in which this wonderful endowment is communicated, or of the relations upon which it is dependent, we are wholly ignorant.
293. The force exerted by the heart is vital. It is distinguished from mechanical force in being produced by the very engine that exerts it. In the best-constructed machinery there is no real generation of power. There is merely concentration and direction of it. In the recoil of the spring, in the reaction of condensed steam, the energy of the expansive impulse is never greater than the force employed to compress or condense, and the moment this power is expended all capacity of motion is at an end. But the heart produces a force equal to the pressure of sixty pounds by the gentlest application of a bland fluid. Here no force is communicated to be again givenout, as in every mechanical moving power; but it is new power, power really and properly generated; and this power is the result of vital action, and is never in any case the result of action that is not vital.
294. The heart projects the blood with a given force into the arterial tubes. The arteries in the living body are always filled to distension, and somewhat beyond it, by the quantity of blood that is in them. It has been shown that the elasticity of their coats is such as to give to them, even after death, the form of open hollow cylinders (274). During life they are kept in a state of distension by the quantity of blood they contain. By virtue of their elasticity they react upon their contents with a force exactly proportioned to the degree of their distension, that is, with a force at least adequate to keep them always open and rigid.
295. These open and rigid tubes, already filled to distension, and somewhat beyond it, receive at every contraction of the heart a forcible injection of a new wave of blood. The first effect of the injection of this new wave into a tube previously full to distension, is to cause the current to proceed by jerks or jets, each jerk or jet corresponding to the contraction of the heart. And, accordingly, by this jet-like motion, the flow of the blood in the artery is distinguished from that in the vein, in which latter vessel the current is an equal and tranquil stream.
296. The second effect of this new wave is to occasion some further distension of the already distended artery, and accordingly, when the vessel is exposed in a living animal, and its action carefully observed, a slight augmentation of its diameter is distinguishable at every contraction of the heart. This new wave while it distends must at the same time slightly elongate the vessel; cause its straight portions to bend a little, and its curved portions to bend still more; and, consequently, in some situations, to lift it a little from its place, giving it a slight degree of locomotion;—and these two causes combined produce the pulse. When the finger is pressed gently on an artery, at the instant of the contraction of the heart, the vessel is felt to bound against the finger with a certain degree of force: this, as just stated, is owing to a slight distension of the vessel by the new wave of blood, together with a slight elongation of it, and a gentle rising from its situation.
297. The blood, in flowing through the arterial trunks and branches to the capillaries, through the arterial to the venous capillaries, and through the venous branches and trunks back to the heart, is exposed to numerous and powerful causes of retardation: such, for example, as the friction between the blood and the sides of the vessels, the numerous curves and angles formed by the branches in springing from the trunks, the tortuous course of the vessels in many parts of the body, and theincreasing area of the arterial branches as they multiply and subdivide. Yet the extraordinary fact has been recently discovered, that the blood moves with the same momentum or force in every part of the arterial system, in the aorta, in the artery in the neck which carries the blood to the head (the carotid artery), in the artery of the arm (the humeral artery), in the artery of the lower extremity (the femoral artery); in a word, in the minute and remote capillary, and in the large trunk near the heart. Having contrived an instrument by which the force of the blood as it flows in its vessel could be accurately indicated by the rise of mercury in a tube, M. Poiseuille found that the elevation of the mercury is uniformly the same in the different arteries of the same animal, whatever the size of the artery and its distance from the heart. This tube was inserted, for example, into the common carotid artery of a horse: the diameter of the vessel was 34/100ths of an inch; its distance from the heart was thirty-nine inches; the height to which the mercury rose in the graduated tube was accurately marked. The tube was then inserted into a muscular branch of the artery in the thigh: the diameter of this vessel was 7/100ths of an inch, and its distance from the heart 67½ inches. According to the mean of nine observations, the mercury rose in both tubes to precisely the same elevation. Here is another instance of the beautiful adjustments everywhere establishedin the living economy. The blood is sent by a living engine, moving under laws peculiar to the state of life, into living vessels, which in their turn acting under laws peculiar to the state of life, so accommodate themselves to the current as absolutely to offer no resistance to its progress; so accommodate themselves to the moving power, as completely and everywhere to obviate the physical impediments to motion inseparable from inorganic matter.
298. That the arterial tubes do possess and exert a truly vital power, modifying the current of the blood they contain, is indubitably established.
1. If in a living animal the trunk of an artery be laid bare, the mere exposure of it to the atmospheric air causes it to contract to such a degree, that its size becomes obviously and strikingly diminished. This can result only from the exertion of a vital property, for no dead tube is capable in such a manner of diminishing its diameter.
2. If during life an artery be opened and the animal be largely bled, the arteries become progressively smaller and smaller as the quantity of blood in the body diminishes. If the bleeding be continued until the animal dies, and the arteries of the system be immediately examined, they are found to be reduced to a very small size; if again examined some time after death, they are found to have become larger, and they go on growing successively larger and larger until they regain nearlytheir original magnitude, which they retain until they are decomposed by putrefaction.
3. M. Poiseuille distended with water the artery of an animal just killed. This water was urged by the pressure of a given column of mercury. The force of the reaction of the artery was now measured by the height of a column of mercury which the water expelled from the artery could support. It was found that the artery reacted with a force greater than that employed to distend it, and greater than the same artery could exert some time after death; but since mechanical reaction can never be greater than the force previously exerted upon it (293), it follows that the excess of the reaction indicated in this case was vital.
4. If an artery be exposed and a mechanical or chemical stimulus be applied to it, its diameter is altered, sometimes becoming larger and sometimes smaller, according to the kind of agent employed.
299. Any one of these facts, taken by itself, affords a demonstration that the arterial trunks and branches are capable of enlarging and diminishing their diameter by virtue of a vital endowment. There is complete evidence that the exertion of this vital power on the part of the arterial trunk is not to communicate to the blood the smallest impulsive force; the engine constructed for the express purpose of working the current generates all the force that is required; but the labour of the engine is economized by impartingto the tubes that receive the stream a vital property, by which they wholly remove the physical obstructions to its motion.
300. Driven by the heart through the arterial branches into the capillaries, the blood courses along these minute vessels urged by the same power. The most careful observers, from Haller and Spalanzani down to the present time, concur in stating that the pulsatory movement communicated by the heart to the blood in the great arteries is distinctly visible under the microscope in the capillaries. "I have often observed in frogs and tadpoles, and once in the bat," says Wedemeyer, "that when the circulation was becoming feeble, the blood in the finest capillaries advanced by jerks, corresponding with the contractions of the heart. I remarked the same appearance in the fine veins several times in the toad and tadpole, and once in the frog." If an experimenter so dispose the circulation of the limb of an animal that the flow of blood be confined to the branches of a single artery, and a corresponding vein, it is found that the blood stagnates in the vein whenever the current in the artery is stopped by a ligature, but no sooner is the ligature removed from the artery, than the blood begins again to flow freely along the vein, the capillaries of the artery which have to send on the current to those of the vein being now again within the influence of the heart. And if the impulse of the heart be removed from thecapillary system, by placing a ligature around the aorta, the capillary circulation is uniformly and completely stopped.
301. It was found by Dr. Hales, that, under ordinary circumstances, the blood rises in a tube connected with a vein to the height only of six inches, while it has been shown (290) that in the artery it ascends as high as ten feet. This prodigious difference between the venous and the arterial tension led to the conclusion that the impulsive force of the heart was all but exhausted before the blood reached the veins, and set physiologists on the search for other powers to carry on the venous circulation. It was overlooked that the blood has an open and ready escape from the great trunks of the veins through the right chambers of the heart, and that in consequence of this free escape of their fluid, these vessels indicate no greater tension than is just sufficient to lift the blood to the heart, and to overcome friction[7]. M. Magendie having laid bare the chief artery and vein of a living limb, and having raised the vessels in such a manner that he could place a ligature around the former, without including the latter, found that the flow of blood from a puncture made below a ligature on the vein, was rapid or slow, according as the heart was allowed to produce a greater or less degree of tension in the artery,which tension was regulated by compressing the artery between the fingers. After a similar preparation of a limb, a ligature was placed around the vein; a tube was then inserted into it; it was found that the blood ascended in the tube from the obstructed vein just as high as from the artery.
302. Thus we are able to trace the action of the heart from the beginning to the end of the circle. Of this circle it is the sole moving power; but it is a living engine acting in combination with living vessels. The force it exerts is a vital force, economized by the agency of a vital property communicated to the vessels, by virtue of which they spontaneously and completely remove all physical obstruction to the progress of the stream through its channels.
303. Some German physiologists of great eminence, after a careful and patient observation of the blood, have satisfied themselves that in addition to the contraction of the heart, it is necessary to admit a second original and independent motive force, namely, a self-moving power inherent in the particles of the blood itself. The blood we know is a living substance. No reason can be assigned why the power of originating motion should not be communicated to such a substance as well as to the muscular fibre, of which, indeed, one constituent of the blood affords the basis. Such a power, if found to be inherent in the particles of theblood, would explain some phenomena connected with the circulation not yet clearly elucidated; but the proof of the self-moving power of the blood does not yet seem to be complete. It is, however, impossible to explain the phenomena of the circulation, or to obtain a satisfactory view of some of the other functions of the economy, without supposing the particles of the blood to be endowed with a vital power of repulsion, in consequence of which they are prevented from uniting when in contact, and the fluidity of the mass is maintained.
In this account of the powers that move the blood, no notice has been taken of the physical agents supposed to act as auxiliaries to the heart, in carrying on the circulation, such as the suction power of the thorax, and of the auricles of the heart, and the capillary attraction of the vessels; because, without questioning the existence of such agents, or denying that advantage may be taken of them, it seems pretty clear that their influence is but trivial, and they assumed importance only when the vital endowments of the tissues were not well understood.
304. The ultimate end for which the apparatus of the circulation is constructed, and for which all its action is exerted, is to convey arterial blood to the capillary arteries. These vessels are totally distinct in structure and in office from the larger arterial tubes. All the tunics of these minutevessels diminish in thickness and strength as the tubes lessen in size, but more especially the middle or the fibrous coat; which, according to Wedemeyer, may still be distinguished by its colour in the transverse section of any vessel whose internal diameter is not less than the tenth of a line; but that it entirely disappears in vessels too small and too remote to receive the wave of blood in a manifest jet. But while the membranous tunics diminish, the nervous filaments distributed to them increase: the smaller and thinner the capillary, the greater the proportionate quantity of its nervous matter; and this is most manifest in organs of the greatest irritability. The coats of the capillaries successively becoming thinner and thinner, at length disappear altogether, and the vessels ultimately terminate in membraneless canals formed in the substance of the tissues. "The blood in the finest capillaries," says Wedemeyer, "no longer flows within actual vessels; it is not contained in tubes whose parietes are formed by a membranous substance distinguishable by its texture and compactness from the adjoining cellular tissue: it is contained in the different tissues in channels which it forms in them for itself; and, under the microscope, the stream is seen easily and rapidly to work out for itself a new passage in the tissues which it penetrates."
305. Some of these fine capillaries, before they entirely lose their membranous tunics, communicatedirectly with veins. Of the capillaries which terminate by direct communication with veins, some are large enough to admit of three or four of the red particles of the blood abreast; the diameter of others is sufficient to admit only of one; while others are so small that they can transmit nothing but the serum of the blood. As long as the capillary is of sufficient magnitude to receive three or four of the particles abreast, it is evident that it possesses regular parietes; but by far the greater number, before they communicate with veins, lose altogether their membranous coats. There are no visible openings or pores in the sides or ends of the capillaries by means of which the blood can be extravasated, preparatory to its being imbibed by the veins. There is nowhere apparent a sudden passage of the arterial into the venous stream; no abrupt boundary between the division of the two systems. The arterial streamlet winds through long routes, and describes numerous turns before it assumes the nature and takes the direction of a venous streamlet. The ultimate capillary rarely passes from a large arterial into a large venous branch.
306. The vital power which it has been shown (298) is possessed by the arterial trunks and branches, is still more intense in the minute capillaries. If alcohol, strong acetic acid, naphtha, and other stimulating fluids, be injected into the arteries of a living animal, it is found that theyare not transmitted through the capillaries at all, or, at all events, that they make their way through them with extreme difficulty; whereas mild, unirritating fluids pass with rapidity and ease. Wedemeyer exposed and divided the main artery in the fore-leg of a horse, together with the corresponding vein in the shoulder. Several syringes-full of tepid water were now injected into the lower end of the artery. The gentlest pressure was sufficient to force the fluid through the capillaries. At each injection the water issued in a full stream from the aperture of the vein, the flow of the fluid ceasing as soon as the injection was stopped. Next, instead of water, four syringes-full of pure cold brandy were injected. To propel this fluid through the capillaries, so as to render its smell and taste perceptible at the aperture of the vein, required a great degree of pressure; and when at last the fluid issued from the vein, it merely trickled in a feeble stream.
The experiment being repeated on another horse with vinegar, six syringes-full of which being injected in rapid succession, at first this fluid passed as easily as water, afterwards it flowed with greater difficulty and in a small stream; before long the force required to propel it was extreme, and at last the obstruction to its passage became complete, so that no fluid whatever issued from the vein.
These experiments, whenever repeated, affordedthe same result, and they demonstrate that the capillaries are capable of being stimulated to contract upon their contents, and that they can contract with such force as to stop the current. It is manifest that the power by which they do this is vital, because after death all fluids, the mildest and the most acrid, pass through them with equal facility.
307. Drs. Thompson, Philip, Hastings, and others in this country, have applied stimulants of various kinds to the capillary arteries, in order to observe with the microscope the changes which the vessels undergo. The results of these experiments, performed independently, agree with each other; and all the observers concur in stating that those results are so obvious and decisive as to admit of no question. Wedemeyer, fully aware of all that had been done on this subject by the English physiologists, repeated their experiments with his usual patience and care, vigilantly watching the effects with his microscope. His observations completely coincide with those of our countrymen. The circulation being observed in the mesentery of the frog and in the web of its foot, it was apparent that no change whatever took place in the diameter of the small arteries, nor in that of the capillaries, as long as the circulation was allowed to go on in its natural state; but as soon as stimulants were applied to them, an alteration of their diameter was visible. Alcohol,without much apparent contraction of the vessels, stopped the flow of the blood. Muriate of soda, in the course of three or four minutes, caused the vessels to contract one-fifth of their calibre, which contraction was followed by dilatation and gradual retardation and stoppage of the blood. Ammonia caused immediate and direct dilatation, and the effect of galvanism was still more striking. In a space of time varying from ten to thirty seconds, nay, sometimes immediately after the completion of the galvanic circle, the vessels contracted, some a fourth, others half, and others three-fourths, of their calibre. The flow of the blood through the contracted vessels was accelerated. The contraction sometimes lasted a considerable time, occasionally several hours; in other instances the contraction ceased in ten minutes, and the vessels resumed their natural diameter. A second application of galvanism to the same capillaries seldom caused any material contraction.
308. The evidence, then, is abundant that stimulants are capable of modifying to a great extent the action of the capillary arteries, sometimes causing them to contract, at other times to dilate; sometimes quickening the flow of blood through them, at other times retarding it, and frequently altogether arresting its motion. This contractile power of the capillaries must be a vital endowment, for no such property is possessed by any substance destitute of life, and there is satisfactoryevidence that it is communicated, regulated, and controlled by the organic nerves, which, as has been shown, increase as the size of the vessels and the thickness of their membranous tunics diminish. The powerful influence of these nerves upon the capillary vessels is placed beyond doubt or controversy by the obvious local changes produced in the capillary circulation by sudden, and even by mental, impressions, by the flush of the cheek and the sparkle of the eye, at a thought conceived or a sound heard; changes which can be effected, as far as we have any knowledge, by no medium excepting that of the nerves. The part performed by electricity, the physical agent by which it is conceived the nerves operate, will be considered hereafter.
309. Exerting upon each other a vital force of repulsion, under a vital influence derived from the organic nerves, urged by the vital contraction of the heart, the particles of the blood reach the extreme capillaries. Most of these capillaries terminate (304) in canals, which they work out for themselves in the substance of the tissues. The tissues are endowed with a vital attractive force, which they exert upon the blood—an elective as well as an attractive force: for in every part of the body, in the brain, the heart, the lung, the muscle, the membrane, the bone, each tissue attracts only those constituents of which it isitself composed. Thus the common current, rich in all the proximate constituents of the tissues, flows out to each. As the current approaches the tissue, the particles appropriate to the tissue feel its attractive force, obey it, quit the stream, mingle with the substance of the tissue, become identified with it, and are changed into its own true and proper nature. Meantime, the particles which are not appropriate to that particular tissue, not being attracted by it, do not quit the current, but passing on, are borne by other capillaries to other tissues, to which they are appropriate, and by which they are apprehended and assimilated, When it has given to the tissues the constituents with which it abounded, and received from them particles no longer useful, and which would become noxious, the blood flows into the veins to be returned by the pulmonic heart to the lung, where, parting with the useless and noxious matter it has accumulated, and, replenished with new proximate principles, it returns to the systemic heart, by which it is again sent back to the tissues.
310. Particles of blood are seen to quit the current and mingle with the tissues; particles are seen to quit the tissues and mingle with the current. But all that we can see, with the best aid we can get, does but bring us to the confines of the grand operations that go on, of which we are altogether ignorant. Arterial blood is conveyedby the arteries to the capillaries; but before it has passed from under the influence of the capillaries it has ceased to be arterial blood. Arterial blood is conveyed by the carotid artery to the brain; but the cerebral capillaries do not deposit blood, but brain. Arterial blood is conveyed by its nutrient arteries to bone, but the osseous capillaries do not deposit blood, but bone. Arterial blood is conveyed by the muscular arteries to muscle, but the muscular capillaries do not deposit blood, but muscle. The blood conveyed by the capillaries of brain, bone, and muscle is the same, all comes alike from the systemic heart, and is alike conveyed to all tissues; yet in the one it becomes brain, in the other bone, and in the third muscle. Out of one and the same fluid these living chemists manufacture cuticle, and membrane, and muscle, and brain, and bone; the tears, the wax, the fat, the saliva, the gastric juice, the milk, the bile, all the fluids, and all the solids of the body.
311. And they do still more; for they are architects as well as chemists; after they have manufactured the tissue, they construct the organ. The capillaries of the eye not only form its different membranes and humours, but arrange them in such a manner as to constitute the optical instrument; and the capillaries of the brain not only form cerebral matter, but build it up into the instrument of sensation, thought, and motion.
312. The practical applications of these phenomena are numerous and most important; but they can be clearly and impressively stated only when the operation of the physical agents which influence the circulation, and which proportionally affect life and health, has been explained.