Chapter 4

Fig. CXXXIII.—Respiratory Apparatus of the Bird, as seen in the Swan.1. The Trachea. 2. The lungs. 3. Apertures through which air passes into, 4. Air cells of the body. 5. A bristle passed from one of the air cells of the body, to the cavity containing the lungs. 6. A bristle passed from the cavity of the thigh-bone into another air cell of the body.

Fig. CXXXIII.—Respiratory Apparatus of the Bird, as seen in the Swan.

1. The Trachea. 2. The lungs. 3. Apertures through which air passes into, 4. Air cells of the body. 5. A bristle passed from one of the air cells of the body, to the cavity containing the lungs. 6. A bristle passed from the cavity of the thigh-bone into another air cell of the body.

355. In birds, the next order of vertebrata (fig.CXXXIII.), as in insects, the class of invertebrated animals which are formed for flight (352), the respiratory organs extend through the greater part of the body (fig.CXXXIII.4). The lungs (fig.CXXXIII.2), which still consist of a single pulmonic sac on each side (fig.CXXXIII.2), are divided into cells, minute compared with those of the reptile, yet large compared with those of the quadruped; at the same time numerous air sacs, similar in structure to those of the lungs, but of larger size, are distributed over different parts of the body (fig.CXXXIII.4), which communicate with the air cells of the lungs (fig.CXXXIII.3); while of these larger sacs, several communicate also with the bones (fig.CXXXIII.6), so as to fill with air those cavities which in other animals are occupied with marrow.

356. In the mammalia, the highest order of the vertebrata, respiration is less extended through the system, and is concentrated in a single organ, the lung, which, though comparatively smaller in bulk than in some of the lower classes, is far more developed in structure. The lung in this class consists of a membranous bag, divided into an immense number of distinct vesicles or cells, in the closest possible proximity with each other, yet not communicating, and presenting, from their minuteness, a vast extent of internal surface. Thisbag is confined to a distinct cavity of the trunk, the thorax (fig.CXXXIV.), completely separated from the abdomen by the muscular partition, the diaphragm (fig.CXXXIV.10). This organ nolonger sends down cells into the abdomen, nor membranous tubes into the bones; but is concentrated within the thorax along with the heart (fig.CXXXIV.2, 3, 8). In all the orders of this class, the development and concentration of the organ are in strict proportion to the perfection of the general organization.

Fig. CXXXIV.—View of the Respiratory Apparatus in Man.1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures, dividing each lung into, 5. Large portions termed lobes. 6. Smaller divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10. Diaphragm separating the cavity of the thorax from that of the abdomen.

Fig. CXXXIV.—View of the Respiratory Apparatus in Man.

1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures, dividing each lung into, 5. Large portions termed lobes. 6. Smaller divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10. Diaphragm separating the cavity of the thorax from that of the abdomen.

357. In man there are two pulmonary bags (fig.CXXXIV.2, 3), of nearly equal size, which, together with the heart, completely fill the large cavity of the thorax (fig.CXXXIV.), their external surface being everywhere in immediate contact with the thoracic walls. One of these bags is placed on the right side of the body, constituting the right lung (fig.CXXXIV.2), and the other on the left, constituting the left lung (fig.CXXXIV.3). Each lung is divided by deep fissures, into large portions called lobes (figs.CXXXIV.4, andCXXXV.6), of which there are three belonging to the right, and two to the left lung. Each lobe is subdivided into innumerable smaller parts termed lobules (figs.CXXXIV.6, andCXXXV.6), while the lobules successively diminish in size until they terminate in minute vesicles that constitute the great bulk of the organ (fig.CXXXV.8).

358. The complete centralization of the respiratory function which thus takes place in man, renders the apparatus exceedingly complex both on account of the expedients which are necessary to obtain the requisite extent of surface, in thesmall allotted space, and to bring into contact within that space the fluids that are to act on each other.

Fig. CXXXV.—View of the Air Tubes and Lung.1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5. Left lung; the fissures denoted by the two lines which meet at 6, dividing it into three lobes, and the smaller lines on its surface marking the division of the lobes into lobules. 7. Large bronchial tubes. 8. Minute bronchial tubes terminating in the air cells or vesicles.

Fig. CXXXV.—View of the Air Tubes and Lung.

1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5. Left lung; the fissures denoted by the two lines which meet at 6, dividing it into three lobes, and the smaller lines on its surface marking the division of the lobes into lobules. 7. Large bronchial tubes. 8. Minute bronchial tubes terminating in the air cells or vesicles.

359. The apparatus consists of a vessel to carry the air to the blood; a vessel to carry the blood to the air; an organ in which the air and the blood meet; and an organization by which both fluids are put in motion. The vessel that carries the air to the blood is the windpipe (fig.CXXXV.1, 2); the vessel that carries the blood to the air is the pulmonary artery (fig.CXL.7); the organ in which the blood and the air meet is the lung (fig.CXXXV.5); the organization which puts the air in motion, is the structure of bones, cartilage and muscles, called the thorax (figs.CXLI.andCXLVI.), and the engine that communicates motion to the blood is the right ventricle of the heart (fig.CXL.5).

360. The windpipe is a tube which extends from the mouth and nostrils to the lung (figs.CLIII.1, 9, andCXXXV.2, 5). It is attached to the back part of the tongue (fig.CLII.2, 9), and passes down the neck immediately before the esophagus, or the tube which leads to the stomach (fig.CLIII.9, 12).

361. In the different parts of its course the windpipe is differently constructed, performs different offices, and receives different names according to the diversity of its structure and function. The first division of it is called the larynx (fig.CXXXV.1.), the second the trachea (fig.CXXXV.2), the third the bronchi (figs.CXXXV.3, 4, 7, andCXXXVII.), and the fourth the air vesicles or cells (figs.CXXXV.8, andCXXXVIII.2).

Fig. CXXXVI.—Posterior View of the Larynx and Trachea.1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4. Arytenoid cartilages, separated from each other. 5. Epiglottis. 6. Opening of the glottis. 7. Termination of the cartilaginous rings of the trachea. 8. The ligamentous portion of the trachea. 9. Trachea laid open, showing its internal mucous surface and follicles, with the anterior portion of the cartilaginous rings appearing through it.

Fig. CXXXVI.—Posterior View of the Larynx and Trachea.

1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4. Arytenoid cartilages, separated from each other. 5. Epiglottis. 6. Opening of the glottis. 7. Termination of the cartilaginous rings of the trachea. 8. The ligamentous portion of the trachea. 9. Trachea laid open, showing its internal mucous surface and follicles, with the anterior portion of the cartilaginous rings appearing through it.

362. The first portion of the windpipe called the larynx (figs.CXXXV.andCXXXVI.), constitutes the organ of the voice. It is situated at the upper and fore part of the neck (fig.CLIII.7, 9), immediately under the bone to which the root of thetongue, called the os hyoides (figs.CLIII.6, andCXXXVI.1), is attached. The larynx forms a very complex structure, and is composed of avariety of cartilages, muscles, ligaments, membranes, and mucous glands (fig.CXXXVI.2, 3, 4, 5). At its upper part is a narrow opening of a triangular figure called the glottis (fig.CXXXVI.6), by which air is admitted to and from the lung. Immediately above this opening is placed the cartilage, which obtains its name from its situation,epiglottis(fig.CXXXVI.5), which is attached to the root of the tongue (fig.CLIII.6, 7), and which may be distinctly seen in the living body by pressing down the tongue.

363. The Epiglottis is highly elastic, and is an agent of no inconsiderable importance in respiration, deglutition, and speaking. In respiration it breaks the current of air which rushes to the lungs through the mouth and nostrils, and prevents it from flowing to the delicate air cells with too great a degree of force. During the action of deglutition the epiglottis is carried completely over the glottis (fig.CLIII.6, 7, 8), partly because it is necessarily forced backwards, when the tongue passes backwards in delivering the food to the pharynx (fig.CLIII.6, 7, 8, 10), partly because it is carried backwards by certain minute muscles which act directly upon it, and perhaps also partly in consequence of its own peculiar irritability. The moment the action of deglutition has been performed the epiglottis springs from the aperture of the glottis, partly by its own elasticity, and partly by the return of the tongue to its former position.During the act of speaking the column of air which is expelled from the lung, which rushes through the glottis, and which thus forms the voice, strikes against the epiglottis, and the voice becomes thereby in some degree modified.

Fig. CXXXVII.View of the trachea, showing, first, the division of the tube into the right and left bronchus, and the subdivision of the bronchi into the bronchial tubes; and secondly, the membranous and cartilaginous tissues of which the organ is composed.

Fig. CXXXVII.

View of the trachea, showing, first, the division of the tube into the right and left bronchus, and the subdivision of the bronchi into the bronchial tubes; and secondly, the membranous and cartilaginous tissues of which the organ is composed.

364. The second portion of the windpipe termed the trachea (fig.CXXXV.2), commences at the under part of the larynx (fig.CXXXV.1), and extends as far as the third dorsal vertebra, oppositeto which it divides into two branches which are termed the bronchi (fig.CXXXV.3, 4, andCXXXVII.). One of these branches, called the right bronchus, goes to the right lung; the other branch, called the left bronchus, goes to the left lung (fig.CXXXV.3, 4).

365. The trachea of man, like the tracheæ of the air-breathing insect (351), is composed of three tissues. These tissues differ essentially from each other in nature, and are widely different in form and arrangement. They consist of membrane, muscle, and cartilage.

366. The membranous portion of the human trachea consists of three coats, the cellular (fig.CXXXVII.), the ligamentous (fig.CXXXVI.8), and the mucous (fig.CXXXVI.9). From the cellular and ligamentous coats the tube receives its strength, and in some degree its elasticity; and the mucous coat constitutes the chief seat of the respiratory function. Between the ligamentous and mucous coats are placed two sets of muscular fibres; the first, the external set, passes in a circular direction around the tube; the second set, placed immediately beneath the circular, is disposed longitudinally, and collected into bundles. The office of the circular fibres is to diminish the calibre of the tube, and that of the longitudinal is to diminish its length.

367. As the tracheæ of the insect are kept constantly open for the free admission of air by theirmiddle membranous tunic, dense, firm, elastic, and coiled into a spiral (351), so, for the accomplishment of the same purpose, there are placed between the membranous coats of the human trachea delicate rings of the more highly organized substance, cartilage (35). These cartilaginous rings amount in the entire course of the tube to sixteen or eighteen in number (fig.CXXXV. 2); each cartilage being about a line in breadth, and the fourth of a line in thickness. They never form complete circles, but only a large segment of a circle (fig.CXXXVI.7); the circle is incomplete behind (fig.CXXXVI.7, 9), because there the esophagus is in direct contact with the trachea (fig.CLIII. 9, 12), and instead of dense and firm cartilage, a soft and yielding substance is placed in this situation, in order that there may be no impediment to the free dilatation of the esophagus during the passage of the food.

368. The point at which the bronchi enter the substance of the lung is called the root of the lung (fig.CXXXV. 3, 4). As soon as the bronchi begin to divide and ramify within the lung each cartilage, instead of preserving its crescent shape, is divided into two or three separate pieces, which nevertheless are still so disposed as to keep the tube open. With the progressive diminution in the size of the bronchial branches, their cartilages become less numerous, and are placed at greater distances from each other, until at length as thebronchi terminate in the vesicles, the cartilages wholly disappear; and with the decreasing number and size of the cartilages, the thickness of the cellular, ligamentous, and muscular coats of the bronchi also lessens, until at the points where the cartilages disappear, the muscular and mucous tunics, now reduced to a state of extreme tenuity, alone remain. The essential constituent of the air vesicles, then, is the mucous membrane; but there is reason to suppose that the muscular tunic is likewise continued over these vesicles.

369. It has been stated that the tracheæ of the insect terminate in the different tissues of its body by minute vesicles of an oblong form. The termination of the bronchi in the human lung presents a strikingly analogous appearance. Malpighi, who with extraordinary talent and success devoted his life to the investigation of the minute structures of the various organs of the human body, represents the mucous membrane of the bronchial tubes as terminating in minute vesicles of unequal size: and Reisseissen, who has more recently resumed the inquiry and examined this structure with extreme care, agrees with Malpighi in stating that the bronchial tubes at their terminal points expand into minute, delicate, membranous vesicles of a cylindrical and somewhat rounded figure (fig.CXXXVIII.2). The bronchial tubes do not divide to any great degree of minuteness (fig.CXXXVIII.1), but terminate somewhat abruptly in the vesicles(fig.CXXXVIII.2), which though minute are large enough to be visible to the naked eye (fig.CXXXVIII.2). Viewed in connexion with the bronchial tubes at their terminal points, the vesicles present a clustered appearance, not unlike clusters of currants attached to their stem (fig.CXXXVIII.2).

Fig. CXXXVIII.—View of the Bronchial Tubes terminating in Air vesicles.Fig. 138.Fig. 139.External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same laid open.

Fig. CXXXVIII.—View of the Bronchial Tubes terminating in Air vesicles.

Fig. 138.Fig. 139.External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same laid open.

Fig. 138.Fig. 139.

External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same laid open.

370. In the insect, for the reason assigned (351), these vesicles are diffused over the system, aërating every point of the body; in man they are concentrated in the lung; yet by their minuteness, and by the mode in which they are arranged, they present in the small space occupied by this organ, so extended a surface that Hales, representing the size of each vesicle at the 100dth part of an inch in diameter, estimates the amount of surface furnished by them collectively at 20,000 squareinches. Keil estimating the number of the vesicles at 174,000,000, calculates the surface they present, at 21,906 square inches. Leiberkuhn at 150 cubic feet; and, according to Monro, it is thirty times the surface of the human body.

Fig. CXL.1. The trachea. 2. The right and left bronchus; the left bronchus showing its division into smaller and smaller branches in the lung, and the ultimate termination of the branches in the air vesicles. 3. Right auricle of the heart. 4. Left auricle. 5. Right ventricle. 6. The aorta arising from the left ventricle, the left ventricle being in this diagram concealed by the right. 7. Pulmonary artery arising from the right ventricle and dividing into, 8. The right, and 9. The left branch. The latter is seen dividing into smaller and smaller branches, and ultimately terminating on the air vesicles. 10. Branches of one of the pulmonary veins proceeding from the terminations of the pulmonary artery on the air vesicles, where together they form the net-work of vessels termed the Rete Mirabile. 11. Trunk of the vein on its way to the left auricle of the heart. 12. Superior vena cava. 13. Inferior vena cava. 14. Air vesicles magnified. 15. Blood-vessels distributed upon them.

Fig. CXL.

1. The trachea. 2. The right and left bronchus; the left bronchus showing its division into smaller and smaller branches in the lung, and the ultimate termination of the branches in the air vesicles. 3. Right auricle of the heart. 4. Left auricle. 5. Right ventricle. 6. The aorta arising from the left ventricle, the left ventricle being in this diagram concealed by the right. 7. Pulmonary artery arising from the right ventricle and dividing into, 8. The right, and 9. The left branch. The latter is seen dividing into smaller and smaller branches, and ultimately terminating on the air vesicles. 10. Branches of one of the pulmonary veins proceeding from the terminations of the pulmonary artery on the air vesicles, where together they form the net-work of vessels termed the Rete Mirabile. 11. Trunk of the vein on its way to the left auricle of the heart. 12. Superior vena cava. 13. Inferior vena cava. 14. Air vesicles magnified. 15. Blood-vessels distributed upon them.

371. Such is the structure of the vessel that carries the air to the blood, and such is the mode of its distribution.

The vessel that conveys the blood to the air is the pulmonary artery, the great vessel which springs from the right ventricle of the heart (fig.CXL.5).

The pulmonary artery soon after it issues from the right ventricle of the heart divides into two branches (fig.CXL. 7, 8, 9), one for each lung (fig.CXL.8, 9). Each branch of the pulmonary artery as soon as it enters its corresponding lung (fig.CXL. 9) divides and ramifies through the organ in a manner precisely similar to the bronchial tubes. Every branch of the artery is in contact with a corresponding branch of the bronchus (fig.CXL. 2), divides as it divides, and accurately tracks its course throughout (fig.CXL. 2), until the ultimate divisions of the artery at length reach the ultimate vesicles of the bronchus (fig.CXL. 2, 10), upon the delicate walls of which the capillary arteries rest, expand, and ramify, forming a net-work of vessels, so complex that the anatomist who first observed it, named it theRete Mirabile, the wonderful net-work, andit is still called theRete Mirabile Malpighi, or theRete Vasculosum Malpighi(fig.CXL.2, 9, 10).

372. The blood which has finished its circulation through the system, returned by the great systemic veins (fig.CXL.12, 13), to the right side of the heart (fig.CXL.3), is driven by the right ventricle (fig.CXL.5), into the pulmonary artery (fig.CXL.7); by the branches of which (fig.CXL.8, 9) it is distributed to the air vesicles of the lungs: consequently the right heart of man bears precisely the same relation to the lungs, that the single heart of the fish bears to the branchiæ; the former is a pulmonic, as the latter is a branchial heart; one half of the double heart of the more highly organized creature is employed to circulate the venous blood of the system through the lungs, as the whole of the single heart of the less highly organized animal, is employed to propel the blood through the branchiæ (368). From the capillary branches of the pulmonary artery in the Rete Mirabile (fig.CXL.9), arises another set of vessels termed the pulmonary veins (fig.CXL.10), which receive the blood from the venous vessels spread out on the air vesicles: for the pulmonary artery is functionally a vein, since it contains venous blood, though it is nominally an artery because it carries blood from the heart (269); and in like manner the pulmonary veins are functionally arteries since they contain arterial blood, though they are nominally veins because they carry blood to the heart (272). The branches of the pulmonary arteries are larger in size and greater in number than those of the pulmonary veins, the reverse of what is observed in any other part of the body; because the pulmonary artery contains the blood which is to be acted upon by the air, while the pulmonary veins merely receive the blood which has been acted upon by the air, and the former ramifies more minutely than the latter, in order that the air may act on a larger surface of blood.

373. In the Rete Mirabile the junction of the air-vessel with the blood-vessel is accomplished. The combination of these two sets of vessels constitutes the lung; for the lung is composed of air-vessels and blood-vessels united, and sustained by cellular tissue, and inclosed in the thin but firm membrane called the pleura (104 and 105).

374. Such is the arrangement of that part of the respiratory apparatus which contains the fluids that are to act on each other. The object of the remaining portion of it is to produce the movements which are necessary to bring the fluids into contact. This is accomplished by the mechanism and action of the thorax and diaphragm (figs.CXLI.andCXXXIV.10).

375. These organs, which invariably act in concert, are so constructed and disposed, that when in action they give to the chest two alternate motions, one that by which its capacity is enlarged; and the other that by which it is diminished. These alternate movements are called the motions of respiration. The motion by which the capacity of the chest is enlarged is termed the action of inspiration, and that by which it is diminished the action of expiration.

376. The action of inspiration, or that by which the capacity of the chest is enlarged, is effected by the combined movements of the thorax and diaphragm; by the ascent of the thorax and by the descent of the diaphragm.

377. The osseous portion of the thorax, which has been fully described (69et seq.), consists of the spinal column (fig.CXLI.1), the ribs with their cartilages (fig.CXLI.2, 3), and the sternum (fig.CXLI.4). The soft portion of the thorax consists of muscles and membrane (figs.CXLII.,CXLVI., andCXLVII.), together with the common integuments of the body. The chief boundaries of the cavity of the thorax before, behind, and at the sides, are osseous, being formed before by the sternum and the cartilages of the ribs (fig.CXLI.4, 3); behind by the spinal column and the necks of the ribs (fig.CXLI.1, 2); and at the sides by the bodies of the ribs. Below the boundary is muscular, being formed by the diaphragm (fig.CXLIII.3).

378. Externally the thorax is convex and enveloped by muscle and skin; internally it is concave (fig.CXLIII.1), and lined by a continuation of the same membrane which envelops the lungs, the pleura (104). But that portion of the pleura which lines the internal wall of the thorax is called the costal pleura (pleura costalis), in contradistinction to that which envelops the lungs, which is termed the pulmonary pleura, or pleura pulmonalis (104). By the costal pleura, a thin but firm and strong membrane, smooth, polished, and like all the membranes of its class (serous membrane 30,et seq.), kept in a state of perpetual moisture and suppleness, by a fluid secreted at its surface, the movements of the thorax are facilitated, at the same time that they are prevented from injuring the delicate organs contained in it.

379. The moveable parts of the osseous portion of the thorax are the ribs and sternum. The ribs, though by one extremity tied with exceeding firmness to the spinal column by ligaments specially constructed, and admirably adapted for that purpose (figs.LVI.1, andLVII.1), and though attached at their other extremity by their cartilages to the sternum (fig.LVIII.), are capable of three motions, an upward, an outward, and a downward motion.

Fig. CXLI.—View of the osseous portion of the Thorax.1. Spinal column. 2. Ribs. 3. Cartilages of ribs. 4. Sternum.

Fig. CXLI.—View of the osseous portion of the Thorax.

1. Spinal column. 2. Ribs. 3. Cartilages of ribs. 4. Sternum.

380. The ribs form a series of moveable arches, the convexity of the arches being outwards, andthe whole being disposed in an oblique direction (fig.CXLI.2). The first rib springs from the vertebral column at nearly a right angle (fig.CXLI.2); the acuteness of this angle increases in succession as the ribs descend from the first to the last (fig.CXLI.2); in this manner each rib is inclined obliquely outwards and downwards, and the obliquity thus given to the general direction of the ribs augments progressively from above downwards (fig.CXLI.2).

381. In consequence of this conformation and arrangement of the ribs, every degree of motion which is communicated to them, necessarily influences the capacity of the space they enclose. If they are moved upwards they must enlarge that space at the sides, because the intervals between each other will be increased (fig.CXLI.2); and from behind forwards, because the distance between the spinal column and the sternum (the sternum being protruded forwards with their cartilaginous extremities) (fig.CXLI.3, 4), will be increased. If, on the other hand, they are moved downwards, the capacity of the thorax will be proportionally diminished in every direction (fig.CXLI.).

Fig. CXLII.View of the intercostal muscles which fill up the interspaces between the ribs. These muscles consist of a double layer of fibres, the external and the internal, which cross or intersect each other.

Fig. CXLII.

View of the intercostal muscles which fill up the interspaces between the ribs. These muscles consist of a double layer of fibres, the external and the internal, which cross or intersect each other.

382. One part of the action of inspiration consists, then, of this ascent of the ribs. The ascent of the ribs is effected by the contraction of a double layer of muscles called the intercostal (fig.CXLII.), placed in succession between each rib; and which communicate this motion in the following mode. The first rib is fixed; the second rib is moveable, but less moveable than the third, the third than the fourth, and so on through the series: consequently the contraction of the intercostal muscles (figs.CXLII.andCXLVI.2) mustelevate the whole series, because the upper ribs afford fixed points for the action of the muscles; and so, when all these muscles contract together, they necessarily pull the more moveable arches upwards towards the more fixed (figs.CXLI.andCXLVI.2).

383. But from the oblique direction of the ribs, they cannot ascend without at the same time protruding forwards their anterior extremities (fig.CXLI.). Those extremities being attached to the sternum, which forms the anterior wall of the thorax, they cannot be protruded forwards without at the same time carrying the sternum forwards with them (fig.CXLI.). Thus, by this two-fold motion of the ribs, an upward and consequently an outward motion, the capacity of the thorax is increased from behind forwards, that is, in its small diameter.

384. Such is the part of the action, in inspiration, performed by the motion of the ribs. The remaining part of that action, by far the most important, consists of the enlargement of the capacity of the thorax from above downwards, or in its long diameter. This is effected by the descent of the diaphragm (fig.CXLIII.).

385. The diaphragm is a circular muscle, forming a complete but moveable partition between the thorax and the abdomen (figs.CXXXIV.10, andCXLIII.3). When not in action its upper surfaceforms an arch (figs.CXLIII.4, andCXLV.1), the convexity of which is towards the thorax (figs.CXLIII.4, andCXLV.1), and reaches as high as the fourth rib (fig.CXLV.1); its under surface, or that towards the abdomen, is concave (figs.CXXXIV.10, andCXLV.1). Its central portion is tendinous (fig.CXLIII.4). This central tendinous portion of the diaphragm, which is in apposition with the heart (fig.CXXXIV.8), and firmly attached to the pericardium (fig.CXXXIV.7), is nearly if not quite immoveable: it is only the lateral or muscular portions (fig.CXLIII.4) that are capable of motion. Its central portion is constructed of dense and firm tendon, and is immoveable, primarily, in order to afford one of the two fixed points (the ribs affording the other fixed point), essential to the action of the muscular fibres that constitute its lateral or moveable portions; and secondarily, in order to afford a support to the heart, which rests upon this central tendon. Thus, in consequence of this tendon being rendered absolutely fixed, the motions of the diaphragm are completely prevented from incommoding the motions of the heart; the function of respiration from interfering with the function of the circulation.

Fig. CXLIII.—View of the Diaphragm.1. Cavities of the thorax. 2. Portion of cavity of the abdomen. 3. Lateral or muscular and moveable portions of the diaphragm. 4. Central or tendinous and fixed portion of the diaphragm.

Fig. CXLIII.—View of the Diaphragm.

1. Cavities of the thorax. 2. Portion of cavity of the abdomen. 3. Lateral or muscular and moveable portions of the diaphragm. 4. Central or tendinous and fixed portion of the diaphragm.

386. During the action of inspiration the muscular or lateral portions of the diaphragm contract (fig.CXLIII.3); its muscular fibres shortenthemselves, and are approximated towards the central tendon (fig.CXLIII.2); the consequence is that the whole muscle descends (fig.CXLIV.1); passes from the fourth to below the seventh rib (fig.CXLIV.), loses its arched form and presents the appearance of an oblique plane (fig.CXLIV.). At the same time the muscles of the abdomen are protruded forwards (fig.CXLIV.2),and the viscera contained in its cavity are pushed downwards. The result of these movements is, that the capacity of the thorax is enlarged by all the space that intervenes between the fourth rib(fig.CXLV.1), and the lowest point of the oblique plane formed by the diaphragm (fig.CXLIV.1), together with all that gained by the protrusion of the walls of the abdomen and the descent of its viscera (fig.CXLIV.2).

Back to Index Next