Chapter 10

Fig. CLXXI.1. Esophagus. 2. Stomach. 3. Liver raised, showing the under surface. 4. Duodenum. 5. Small intestines, consisting of—6. Jejunum and ilium. 7. Colon. 8. Urinary bladder. 9. Gall bladder. 10. Abdominal muscles divided and reflected.

Fig. CLXXI.

1. Esophagus. 2. Stomach. 3. Liver raised, showing the under surface. 4. Duodenum. 5. Small intestines, consisting of—6. Jejunum and ilium. 7. Colon. 8. Urinary bladder. 9. Gall bladder. 10. Abdominal muscles divided and reflected.

662. The first portion of the small intestine is termed the duodenum (fig.CLXVII. 3). It is about twelve inches in length, and, unlike the stomach, which is capable of considerable motion, it isclosely tied down to the back by the peritoneum, which imperfectly covers it. The rest of the small intestine is divided into two portions—theupper two-fifths of which are termed jejunum, and the three lower ilium.

663. The duodenum, the chamber which receives the chyme from the pylorus, is a second stomach, which carries on the process commenced in the first. It is assisted in the performance of its function by two organs of considerable magnitude, the pancreas and the liver.

664. The pancreas is a conglomerate gland (fig.CLXXII. 5), of an elongated form, placed in the epigastric region, lying transversely across it, immediately behind the stomach (fig.CLXXII. 1), and resting upon the spinal column (fig.CLXXII. 5). Its right extremity is attached to the duodenum (fig.CLXXII. 9), and its left to the spleen (fig.CLXXII. 4). In external appearance it resembles the salivary glands, but it is of much larger size, and its weight, from four to six ounces, is three times greater than that of all the salivary glands together. It secretes a peculiar fluid called the pancreatic juice, which is carried into the duodenum by a tube named the pancreatic duct (fig.CLXVII. 7), which opens into the duodenum about four or five inches from its pyloric end (fig.CLXVII. 2).

Fig. CLXXII.1. Stomach raised. 2. Under surface of liver. 3. Gall bladder. 4. Spleen. 5. Pancreas. 6. Kidneys. 7. Ureters. 8. Urinary bladder. 9. Portion of intestine called duodenum. 10. Portion of intestine called rectum. 11. Aorta.

Fig. CLXXII.

1. Stomach raised. 2. Under surface of liver. 3. Gall bladder. 4. Spleen. 5. Pancreas. 6. Kidneys. 7. Ureters. 8. Urinary bladder. 9. Portion of intestine called duodenum. 10. Portion of intestine called rectum. 11. Aorta.

665. The liver, the largest and heaviest gland in the body, weighing about four pounds, is placed chiefly in the right hypochondriac region (fig.CLXXI. 3); but a portion of it extends transversely across the epigastric, into the left hypochondriacregion (figs.CV.andCVII.3). Its upper surface is in contact with the diaphragm (fig.LX.6, b); its under surface with the pyloric extremity of the stomach (fig.LX.7), and its margin can be felt under the edges of the ribs of the right side.

666. It has been stated (473, 1.) that the fluid secreted by the liver, unlike that formed by any other organ of the body, is elaborated from venous blood, derived from the veins of the digestive organs, and that these veins uniting together, form a common trunk called the vena portæ, which penetrates the liver and ramifies through it in the manner of an artery. Galen long ago compared this venous system to a tree whose roots are dispersed in the abdomen, and its branches spread out through the liver. Two comparatively small arteries, called the hepatic, nourish the liver; the ultimate divisions of these arteries likewise terminate in the vena portæ. The ultimate branches of the vena portæ terminate partly in a system of veins, called the hepatic, which like ordinary veins return the blood to the right side of the heart; and partly in a system of tubes, termed the biliary ducts, which contain the fluid secreted by the capillary branches of the vena portæ. This fluid is the bile. The biliary ducts uniting from all parts of the liver by innumerable branches, at length form a single trunk termed the hepatic duct (fig.CLXVII.9), which carries the bile partly to the gall bladder (fig.CLXVII.8)by a duct called the cystic (fig.CLXVII.10), and partly to the duodenum (fig.CLXVII.3) by a duct named the choledoch (fig.CLXVII.6), a common trunk formed by the union of the cystic with the hepatic (fig.CLXVII.10 and 9). The choledoch duct opens into the duodenum at the same point as the pancreatic (fig.CLXVII.7), and generally by a common orifice.

667. The duodenum, on receiving the chyme from the stomach, transmits it slowly along its surface. The kind of motion by which the chyme is borne along the surface of the duodenum is perfectly analogous to that by which it is transmitted from the stomach to the duodenum, irregular, sometimes in one direction, and sometimes in another, at one time commencing in one part of the organ, at another time in another, always slow, but ultimately progressive.

668. As the chyme slowly advances through the upper part of the duodenum, the biliary and the pancreatic juices slowly distil into the lower portion of the organ. The bile is seen to exude from the choledoch duct, not continually, but at intervals, a drop appearing at the orifice, and diffusing itself over the neighbouring surface, about twice in a minute, while the flow of the pancreatic juice is still slower.

669. No appreciable change takes place in the chyme until it reaches the orifice of the choledoch duct; but as soon as it comes in contact with thisportion of the duodenum, the chyme suddenly loses its own sensible properties, and acquires those of the bile, especially its colour and bitterness. But these properties are not long retained; a spontaneous change soon takes place in the compound. It separates into a white fluid and into a yellow pulp. The white fluid is the nutritive part of the aliment; the yellow pulp is the excrementitious matter.

670. This white fluid, the proper product of the digestive process, as far as it has yet advanced, is called chyle. If any portion of oil or fat have been contained in the food, the chyle is of a milk-white colour; if not, it is nearly transparent. It is of the consistence of cream, and it bears a close resemblance to cream in its sensible properties. It differs from chyme in being of a whiter colour, more pellucid, and of a thicker consistence: it differs also in its chemical nature, for, whereas chyme is acid, chyle is alkaline.

671. Three fluids are mixed with the chyme in the duodenum, each of which contributes to the conversion of the chyme into chyle. First, the secretion of the duodenum itself, a solvent analogous to the gastric juice. Secondly, the secretion of the pancreas, a watery fluid holding in solution highly important principles, namely, a large quantity of albumen, a matter resembling casein, osmazome, and different salts. Thirdly, the secretion of the liver, a compound fluid, consisting of water, mucus, and several peculiar animal matters,namely, resin, cholesterine, picromel, cholic acid, a colouring matter, probably salivary matter, osmazome, casein, and many salts.

672. There cannot be a question that the secretion of the duodenum has a solvent power over the chyme analogous to that of the gastric juice. Some physiologists indeed maintain that the juice poured out from the inner surface of the duodenum is as powerful a solvent as the gastric juice. It is certain that substances which have escaped chymification in the stomach undergo that process in the duodenum, and that there is the closest analogy between the action of the duodenum on the chyme and that of the stomach on the crude food.

673. The pancreatic secretion adds to the chyme richly azotized animal substances, albumen, casein, osmazome (671), by which it is brought nearer the chemical composition of the blood, and prepared for its complete assimilation into it. The first addition of such assimilative matter, it has been shown, is communicated by the salivary glands, but far more important additions are now supplied from the pancreas. Hence the larger size of the pancreas and the more copious secretion of the pancreatic fluid, in herbivorous than in carnivorous animals; hence the change produced in the size of the pancreas by a long continued change in the habits of an animal; hence the smaller size of the pancreas in the wild cat, which lives wholly on animal food, than in the domestic cat, whichlives partly on animal and partly on vegetable food.

674. The bile, the most complex secretion in the body, accomplishes manifold purposes.

1. Like the pancreatic secretion, it communicates to the chyle richly azotized animal substances, picromel, osmazome, and cholic acid (671); by the combination of which with the chyme, it is brought still nearer the chemical composition of the blood. These principles are manifestly united with the chylous portion of the chyme, since they are not discoverable in its excrementitious matter.

2. Bile has the property of dissolving fat; consequently, when oily or fatty matters are contained in the food, it powerfully assists in converting these substances into chyle.

3. The excrementitious portion of the bile is highly stimulant. The contact of its bitter resin with the mucous membrane of the intestines excites the secretion of that membrane; hence the extreme dryness of the excrementitious matter when the choledoch duct of an animal has been tied; and hence the same dryness of this matter in jaundice, when the bile, instead of being conveyed by its appropriate duct into the duodenum, is taken up by the absorbents, poured into the blood, and distributed over the system.

4. The bitter resin of the bile stimulates to contraction the fibres of the muscular tunic of the intestines: by the contraction of these fibres the excrementitious matter is conveyed in due time out of the body; hence the constipated state of the bowels invariably induced when the secretion of the bile is deficient, or when its natural course into the intestines is obstructed.

5. The excrementitious portion of the bile exerts an antiseptic influence over the excrementitious portion of the food during its passage through the intestines. In animals in which the choledoch duct has been tied, the excrementitious portion of the food is invariably found much further advanced in decay than in the natural state. This is also uniformly the case in the human body in proportion as the secretion of the bile is deficient, or its passage to the intestine is obstructed.

675. Such appear to be the real purposes accomplished by the bile in the process of digestion. Several uses have been assigned to it, in promoting this process, which it does not serve. Seeing the instantaneous change wrought in the chyme on its contact with the bile, it was reasonable to suppose that the main use of the bile was to convert chyme into chyle, a purpose apparently of sufficient importance to account for the immense size of the gland constructed for its elaboration. The soundness of this conclusion appeared to be established by direct experiment. Mr. Brodie placed a ligature around the choledoch duct of an animal: after the operation the animal ate as usual: on killing the animal some time after it had taken ameal, and examining the body immediately after death, it was clear that chymification had gone on in the stomach just as when the choledoch duct was sound, but no chyle appeared to be contained either in the intestines or in the lacteals. In the lacteals there was found only a transparent fluid, which was supposed to consist of lymph and of the watery portion of the chyme. Mr. Brodie’s experiments seemed to be confirmed by those of Mr. Mayo, who arrived at the conclusion, that when the choledoch duct is tied, and the animal is examined at various intervals after eating, no trace whatever of chyle is discoverable in the lacteal vessels. But these experimentalists inferred that no chyle existed in the intestines or lacteals, because there was present no fluid of a milk-white colour, a colour not essential to chyle, but dependent on the accident of oily or fatty matter having formed a portion of the food. These experiments have been repeated in Germany by Tiedemann and Gmelin, and in France by Leuret and Lassaigne, who have invariably found, after tying the choledoch duct, nearly the same chylous principles, with the exception of those derived from the bile, as in animals perfectly sound; and the English physiologists have since admitted that their German and French colaborateurs have arrived at conclusions more correct than their own.

676. The bile consists then of two different portions; a highly animalized portion, which combineswith the chyme and exalts its nature by approximating it to the condition of the blood; and an excrementitious portion, which, after accomplishing certain specific uses, is carried out of the system with the undigested matter of the food. The excrementitious portion of the bile, namely, the resin, the fat, the colouring principle, the mucus, the salts, constitute by far the largest portion of it. These constituents of the bile for the most part contain a very large proportion of carbon and hydrogen, and the reasons have been already fully stated (473,et seq.) which favour the conclusion that the elimination of these substances under the form of bile is one most important mode of maintaining the purity of the blood, and that the liver is thus a proper respiratory organ, truly auxiliary to the lungs. It is a beautiful arrangement, and like one of the adjustments of nature, that the bile, the formation of which abstracts from the blood so large a portion of carbon and hydrogen as to maintain the purity of the circulating mass and to counteract its putrescent tendency, acts on the excrementitious portion of the food, always highly putrescent, as a direct and powerful antiseptic; that the very matter which is eliminated on account of the putrid taint it communicates to the blood, on its passage out of the body, stops the putrefaction of the substances which have been ministering to the replenishment of the blood.

677. The chyle, thick, glutinous, and adhesive,attaches itself with some degree of tenacity to the mucous surface of the duodenum. Nevertheless, by the successive contractions of the muscular fibres of the duodenum the fluid is slowly but progressively propelled forwards. The separation of the excrementitious matter becomes more complete, and consequently the chyle more pure as it advances, until, having traversed the course of the duodenum, it enters the second portion of the small intestines, the jejunum.

678. The jejunum, so called because it is commonly found empty, and the ilium, named from the number of its convolutions, on account of their great length, are provided with a distinct membrane to support them, and to retain them in their situation, termed the mesentery.

679. The mesentery is a broad membrane composed of two layers of peritoneum. Between these two layers, at one extremity of the duplicature, is placed the intestines, while the other extremity is attached to the spinal column. The mesentery being much shorter than the intestines, the intestines are gathered or puckered upon the membrane, by which beautiful mechanical contrivance they are held in firm and close contact with each other, yet their convolutions cannot be entangled, nor can they be shaken from their place by the sudden and often violent movements of the body. It sometimes happens, in consequence of disease, that the convolutions of the intestines are glued together by the effusion of lymph, and then the most triflingcauses are capable of producing the severest symptoms of obstruction in the bowels.

680. The internal surface of the small intestines is distinguished,

1. By the number of the mucous glands, which may be seen by a magnifying glass to consist partly of a prodigious number of the minutest follicles, not collected in groups, but equally scattered throughout; and partly of glands of a larger dimension, disposed in groups at particular parts of the canal.

2. By the increase in the number and size of the villi, of which there are about four thousand to the surface of a square inch. Like those of the stomach, the villi of the small intestine are composed of arteries, veins, nerves, and mucous ducts; but to the villi of the small intestine, in length about one-fourth of a line, there is added a new vessel, the absorbent of the chyle, the lacteal (figs. 175 and 176), so named from the milk-like chylous fluid which it contains.

3. By the great extension of the mucous coat obtained by the disposition of the membrane into the folds called valvulæ conniventes (fig.CLXXIII.). These folds, which rarely extend through the whole circle of the intestine, are often joined by communicating folds (fig.CLXXIII.). The folds are broadest in the middle, and narrowest at the extremities (fig.CLXXIII.). In general, they are about a line and a half broad. One edge of the fold is loose,but the other is fixed to the intestine (fig.CLXXIII.). The office of these folds is, first, without increasing space, to extend surface for the distribution of the villi; and, secondly, to retard the flow of the chyle, by opposing to its descent valves so constructed and disposed as, without arresting its progress, to moderate and regulate its course, in order that time may be allowed for its absorption.

Fig. CLXXIII.Internal view of a portion of the jejunum, showing the arrangement of the mucous membrane into valvulæ conniventes.

Fig. CLXXIII.

Internal view of a portion of the jejunum, showing the arrangement of the mucous membrane into valvulæ conniventes.

Fig. CLXXIV.—View of the Outer Coats of the Small Intestine.1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—a.longitudinal fibres.b.Circular fibres.

Fig. CLXXIV.—View of the Outer Coats of the Small Intestine.

1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—a.longitudinal fibres.b.Circular fibres.

681. The onward flow of the chyle through the course of the small intestines is effected by the action of the double layer of muscular fibres, the circular and the longitudinal fasciculi which compose its muscular coat (fig.CLXXIV.). The disposition of the muscular fibres of the alimentary canal in general, and of this part of it in particular, deserves special notice. The ordinary arrangement and action of muscular fibres would not have produced in this case the kind and degree of motion required. The muscular fibres that compose the ventricles of the heart are soaccumulated and disposed, that their contraction originates, and communicates energetic impulse. The muscles of the arm are so accumulated and disposed that their contraction originates the like energetic impulse. Muscles so accumulated in the alimentary canal would have produced motion, indeed, but motion not only not accomplishing the end in view, but directly defeating it. In order to obtain the kind and degree of motion in this case required, the firm and thick muscle is attenuated into minute, delicate, and thready fibres, not concentrated in a bulky mass, so as to obtain by their accumulation a great degree of force; but spread out in such a manner as to form a thin and almost transparent coat. The tender fibres composing this delicate coat, by their contraction,produce two alternate, gentle, almost constant motions, called the peristaltic, from its resemblance to the motion of the earth-worm, and the antiperistaltic. By the peristaltic action motion is begun at once in several parts of the canal. Whenever the chyle is applied in a certain quantity to any part of the intestines, that part contracts, and makes a firm point, towards which the portions both above and below are drawn, by means of the longitudinal fibres which shorten the canal, and at the same time dilate the under part. By the antiperistaltic action, which is the exact reverse of the former, the chyle is turned over and over, and exposed to the orifices of the lacteal vessels; while, by the motion of the chyle forwards and backwards, and backwards and forwards, produced by these two actions constantly alternating with each other, its slow, gentle, but ultimately progressive course is secured.

682. The chyle thus gently moved along the extended surface of the jejunum and ilium, and still in its course acted upon in some degree by the secretions poured out upon the mucous membrane, successively disappears, until at the termination of the ilium (fig.CLXXI.5) there is scarcely any portion of it to be perceived. It is taken up by the vessels termed lacteals.

683. The lacteal vessels (figs. 175 and 176), take their origin on the surface of the villi, by open mouths, too minute to be visible to the nakedeye, but distinguishable under the microscope. These minute, pellucid tubes, wholly countless in number, are composed of membranous coats so thin and transparent that the milky colour of their contents, from which they derive their name, is visible through them, and yet they are firm and strong. They present a jointed appearance (figs.CLXXVI.4, andCLXXVII.7). Each joint denotes the situation of the valves with which they are provided, and which are placed at regular distances along their entire course (fig.CXCII.1 and 2). These valves, which are generally placed in pairs (fig.CXCII.2), consist of a delicate fold of membrane of a semilunar form, one edge of which is fixed to the side of the vessel, while the other lies loose across its cavity (fig.CXCII.2). So firm is this membrane, and so accurately does it perform the office of a valve, that even after death it is capable of supporting a column of mercury of considerable weight without giving way, and of preventing a retrograde course of the fluid. The lacteals are nourished by blood-vessels, and animated by nerves, and it is conceived that they must be provided with muscular fibres, or some analogous tissue, for they are obviously contractile, and it is by this contractile power that their contents are moved. The delicacy and transparency of the vessels, however, render it impossible to distinguish the different tissues which compose their walls.

Fig. CLXXV.View of the inner surface of the ilium as it appears some hours after a meal. 1. The smaller branches of the lacteals, turgid with chyle, covering the surface of the intestine. 2. Larger branches of the lacteals formed by the union of the smaller branches.

Fig. CLXXV.

View of the inner surface of the ilium as it appears some hours after a meal. 1. The smaller branches of the lacteals, turgid with chyle, covering the surface of the intestine. 2. Larger branches of the lacteals formed by the union of the smaller branches.

Fig. CLXXVI.View of the course of the Lacteals.1. The aorta. 2. Thoracic duct. 3. External surface of a portion of small intestine. 4. Lacteals appearing on the external surface of the intestine after having perforated all its coats. 5. Mesenteric glands of the first order. 6. Mesenteric glands of the second order. 7. Receptacle for the chyle. 8. Lymphatic vessels terminating in the receptacle of the chyle, or commencement of the thoracic duct.

Fig. CLXXVI.View of the course of the Lacteals.

1. The aorta. 2. Thoracic duct. 3. External surface of a portion of small intestine. 4. Lacteals appearing on the external surface of the intestine after having perforated all its coats. 5. Mesenteric glands of the first order. 6. Mesenteric glands of the second order. 7. Receptacle for the chyle. 8. Lymphatic vessels terminating in the receptacle of the chyle, or commencement of the thoracic duct.

684. If the mucous coat of the small intestines be examined some hours after a meal, the lacteals are seen turgid with chyle, covering its entire surface (fig.CLXXV.1). These vessels, which are sometimes of such magnitude and in such numbers as entirely to conceal the ramifications of the blood-vessels, unite freely with each other, and form a net-work, from the meshes of which proceed branches which, successively uniting, form branchesof a larger size (fig.CLXXV. 2). These larger branches perforate the mucous coat and pass for some way between the mucous and the muscular tunics: at length they perforate both the muscular and the peritoneal coats, when, from having been on the inside of the intestine, they get on the outside of it (fig.CLXXVI. 3, 4), and are included, like the intestine itself, between the layers of the mesentery. All the different sets of lacteals converging and uniting together, form an exceedingly complicated plexus of vessels within the fold of the mesentery. Radiating from this plexus, the lacteals advance forwards until they reach the glands, called, from their being placed between the fold of the mesentery, the mesenteric (figs.CLXXVI.5 and 6, and clxxvii. 2 and 3); rounded, oval, pale-coloured bodies, consisting of two sets, arranged in a double row (figs.CLXXVI.5 and 6, andCLXXVII.2 and 3); the set nearest the intestine (fig.CLXXVII.2) being considerably smaller than the succeeding set (fig.CLXXVII.3).

Fig. CLXXVII.View of the course of the Thoracic Duct from its origin to its termination. 1. Lacteal vessels emerging from the mucous surface of the intestines. 2. First order ofmesenteric glands. 3. Second order of mesenteric glands. 4. The great trunks of the lacteals emerging from the mesenteric glands, and pouring their contents into—5. The receptacle of the chyle. 6. The great trunks of the lymphatic or general absorbent system terminating in the receptacle of the chyle. 7. The thoracic duct. 8. Termination of the thoracic duct at—9. The angle formed by the union of the internal jugular vein with the subclavian vein.

Fig. CLXXVII.

View of the course of the Thoracic Duct from its origin to its termination. 1. Lacteal vessels emerging from the mucous surface of the intestines. 2. First order ofmesenteric glands. 3. Second order of mesenteric glands. 4. The great trunks of the lacteals emerging from the mesenteric glands, and pouring their contents into—5. The receptacle of the chyle. 6. The great trunks of the lymphatic or general absorbent system terminating in the receptacle of the chyle. 7. The thoracic duct. 8. Termination of the thoracic duct at—9. The angle formed by the union of the internal jugular vein with the subclavian vein.

685. On reaching the first series of glands (fig. clxxvii. 2), the lacteals penetrate the substance of the gland, in the interior of which they communicate with each other so freely, and form such innumerable windings, that the gland seems to consist of a congeries of convoluted lacteals. Emerging from the first series of glands, the lacteals proceed on their course to the second series (fig.,CLXXVII.3), which they penetrate, and in the interior of whichthey present the same convoluted appearance as in the first set. On passing out of this second series of glands, the lacteals unite together, and compose successively larger and larger branches, until at length they form two or three trunks (fig.CLXXVII.4), which terminate in the small oval sac (fig.CLXXVII.5), termed the receptacle of the chyle (receptaculum chyli).

686. In this oval sac or receptacle of the chyle (fig.CLXXVII.5), which rests upon the second or the first lumbar vertebra, also terminate the trunks of the general absorbent vessels of the system (fig. clxxvii. 6), called from thelymphor the pellucid fluid which they contain, lymphatics, as the lacteals are named from the lactitious or milky appearance of their contents.

687. The receptacle of the chyle produced forms the thoracic duct (fig.CLXXVII.7), a canal about three lines in diameter. This tube rests upon the spinal column, ascends on the right side of the aorta, passes through the aortic opening in the diaphragm (fig.CXXXIV.9, 10), and enters into the chest. Here it forms a transparent tube about the size of a crow-quill; it rests upon the bodies of the dorsal vertebræ; it continues to ascend still on the right side of the aorta, until it reaches the sixth or fifth dorsal vertebra, when changing its direction, it passes obliquely over to the left side (fig.CLXXVII.7). From this point it continues its course upwards,on the left side of the neck, as high as the sixth cervical vertebra; when suddenly turning forwards and a little downwards, it terminates itscourse in the angle formed by the union of the internal jugular with the subclavian vein (fig.CLXXVII.8, 9). At its termination in these great venous trunks are placed two valves, which prevent alike the return of the chyle, and the entrance of the blood into the duct (fig.CLXXVIII.).

Fig. CLXXVIII.—Valve at the termination of the Thoracic Duct.1. The Thoracic Duct. 2. Lymphatics entering the duct. 3. The vein laid open, showing the valve at the termination of the duct. 4. The left internal jugular vein. 5. The left subclavian vein. 6. The vein called innominata. formed by the union of the internal jugular and subclavian veins. 7. The right jugular vein. 8. The right subclavian vein. 9. The superior cava formed by the union of the veins above. 10. The inferior cava formed by the union of the veins below. 11. The two venæ cavæ passing to the right auricle of the heart. 12. The heart. 13. The pulmonary artery dividing into right and left branches. 14. The aorta.

Fig. CLXXVIII.—Valve at the termination of the Thoracic Duct.

1. The Thoracic Duct. 2. Lymphatics entering the duct. 3. The vein laid open, showing the valve at the termination of the duct. 4. The left internal jugular vein. 5. The left subclavian vein. 6. The vein called innominata. formed by the union of the internal jugular and subclavian veins. 7. The right jugular vein. 8. The right subclavian vein. 9. The superior cava formed by the union of the veins above. 10. The inferior cava formed by the union of the veins below. 11. The two venæ cavæ passing to the right auricle of the heart. 12. The heart. 13. The pulmonary artery dividing into right and left branches. 14. The aorta.

688. This account of the course of the thoracic duct is a description of the course of the chyle. Performing a double, circuitous, and slow circulation through the minute convoluted tubes of which the double series of mesenteric glands are composed, the chyle, in its receptaculum, is mixed with the contents of the lymphatic vessels, lymph (fig.CLXXVII.6, 5), that is, organic matter brought from every surface and tissue of the body. Both fluids, chyle and lymph, mixed and mingled, flow together into the thoracic duct, by which in the course traced (687) they are poured into the blood, just as the venous torrent is rushing to the heart (fig.CLXXVIII.6, 9, 11).

689. Thus, the final product of digestion, the chyle; particles of organized matter, the lymph; and venous blood, that is, blood which has already circulated through the system commingled, flowtogether to the right heart, by which it is transmitted to the lungs, where all these different fluids are converted into one substance, arterial blood, to be by the left heart sent out to the system for its support.

690. While these processes are going on, another and a very important function is performed by the remaining portion of the alimentary canal. It is the office of this part of the apparatus to carry out of the body that portion of the aliment which is incapable of being converted into chyle. The preparation of the excrementitious part of the aliment for its expulsion constitutes the process of fecation. The organs in which this process is carried on, and by which the excrementitious matter, when duly prepared for its removal, is conveyed from the body, are the large intestines.

691. The large intestines (fig.CLXXIX.) consist of the cæcum, the colon and the rectum (fig.CLXXIX.). The cæcum varies in length from two inches to six; the colon is about five feet in length, and the rectum is about eight inches.

692. The ilium opens into the cæcum (fig.CLXXIX.8, 10), just as the esophagus opens into the stomach. At this point the ilium is elongated, forming two concentric folds which join at their horns, and between the folds are placed a number of muscular fibres. In this manner is constructed a valve, which is termed the valve of the colon. It is placed in a transverse direction across theintestine, and its action as a valve is very complete. It admits of the free passage of the contents of the small intestines into the large, but it prevents the return of any portion of the contents of the latter into the former.

Fig. CLXXIX.—View of the Abdominal Portion of the Digestive Organs.1. Esophagus. 2. Stomach. 3. Spleen. 4. Liver. 5. Gall-bladder with its ducts. 6. Pancreas with its duct. 7. Duodenum. 8. Small intestines. 9. Large intestines dividing into—10. Cæcum. 11. Ascending colon. 12. Arch of the colon. 13. Descending colon. 14. Sigmoid flexure here imperfectly represented. 15. Rectum.

Fig. CLXXIX.—View of the Abdominal Portion of the Digestive Organs.

1. Esophagus. 2. Stomach. 3. Spleen. 4. Liver. 5. Gall-bladder with its ducts. 6. Pancreas with its duct. 7. Duodenum. 8. Small intestines. 9. Large intestines dividing into—10. Cæcum. 11. Ascending colon. 12. Arch of the colon. 13. Descending colon. 14. Sigmoid flexure here imperfectly represented. 15. Rectum.

Fig. CLXXX.Portion of the large intestine, showing the arrangement of the muscular fibres. 1. The longitudinal fibres collected into bands, and forming larger fasciculi. 2. The circular fibres arranged as in the other intestines.

Fig. CLXXX.

Portion of the large intestine, showing the arrangement of the muscular fibres. 1. The longitudinal fibres collected into bands, and forming larger fasciculi. 2. The circular fibres arranged as in the other intestines.

693. The colon is distinguished by its capacious size, its great length, and its longitudinal bands, which consist of strong muscular fasciculi (fig.CLXXIX.11). It is divided into an ascending portion which occupies the right iliac and hypochondriac regions (fig.CLXXIX.11); the transverse portion, called its arch, which is placed directly across the epigastric region (fig.CLXXIX.12), a descending portion which occupies the left hypochondriac region (fig.CLXXIX.13), and a fourth portion, which being curved somewhat like the italic letter S, is called the sigmoid flexure, which occupies the left iliac region (fig.CLXXIX.14). The sigmoid flexure terminates in the last portion of the alimentary canal, called the rectum (fig.CLXXIX.15), which is placed in the hollow of the sacrum, and which follows the curvature of that bone (fig.XLV.5). The circular fibres of the rectum are accumulated at the termination of the bowel to form the internal sphincter of the anus. External to this is placed another set of fibres, which constitute the external sphincter.

694. The mucous membrane of the large intestines is disposed differently from that of the small intestines, and the mucous membrane of the colon still differently from that of the rectum. In the colon the mucous membrane, instead of being disposed in the form of valvulæ conniventes, is so arranged as to divide its whole surface into minute apartments or cells by which the descent of the fecal matter is retarded still more than the descent of the chyle by the valvulæ conniventes. Some particles of chyle do, however, continue to be separated from the fecal matter, even in the large intestines; and in order that nothing may be lost, a few valvulæ conniventes, with their lacteals, appear here also, while the cells of the colon, by retarding the descent of the fecal matter, allow time for the more complete separation and absorption of the chylous particles.

695. In the rectum the mucous membrane is plaited into large transverse folds, which disappear as the fecal matter descends into the bowel, accumulates in it, and distends it; an arrangement which gives to this portion of the intestine its power of distension, so closely connected with our convenience and comfort.

696. As soon as that portion of the alimentary matter which is transmitted to the large intestines reaches the colon it ceases to be alkaline, the distinctive character of the contents of the small intestines, and becomes acid, just as the whole alimentary mass is acid at the commencement of digestion in the stomach. It acquires albumen; its gases are no longer the same, for whereas pure hydrogen is contained in the small intestines, none is ever found in the large, but in the place of it, carbureted and sulphureted hydrogen; and now for the first time it receives its peculiar odour. As it continues to descend, its fluid parts are progressively absorbed, so that it becomes more and more solid, until it reaches the rectum, when it is almost dry. Here the accumulation of it goes on to a considerable extent, the peristaltic action at first excited by the distension of the rectum being, it would appear, counteracted by the contraction of the external sphincter of the anus. When, however, the distension of the bowel reaches a certain point, it produces a sensation which leads to the desire to expel its contents. The bowel is now thrown into action by an effort of the will, and that action is powerfully assisted by the descent of the diaphragm and the contraction of the abdominal muscles, actions also induced by an effort of the will. Thus the action of the first part of the digestive apparatus, that which is connected with the reception and partly with the deglutitionof the food, is attended with consciousness, and is placed under the control of the will; the main portion of the digestive apparatus, that in which the essential part of the digestive process is carried on, is without consciousness, and is placed beyond the influence of volition; the last portion of the digestive apparatus, that connected with the expulsion of the non-nutrient portion of the aliment, again acquires sensibility and consciousness, and is placed under the control of the will. The striking differences in the arrangement of the muscular fibres in these different parts of the apparatus, in accordance with the widely different function performed by them; the powerful muscles connected with the prehension, mastication and deglutition of the food; the delicate and transparent tissue of fibres forming the muscular coat of the stomach and small intestines; the increase in the number and strength of the fibres of the large intestines, and the prodigious accession to them in the rectum, are adjustments not only exquisite and admirable in their own nature, but so indispensable to our well-being and comfort, that were the appropriate action of either to be suspended but for a short period, life would be extinguished, or if it could be protracted, it would be changed into a state of unbearable torment.

697. From the preceding account of the structure and action of the apparatus of digestion, on a comparison of all the phenomena, it appears that thesuccessive stages of the process are marked by the progressive approximation of the food to the nature of the blood. The main constituents, of the blood are albumen, fibrin, an oily principle, and red particles. Even in the chyme there are traces of albumen, with globules, not indeed to be compared in number with the red particles of the blood, smaller in size, and without colour, but still of an analogous nature. In the chyle of the duodenum the quantity of albumen is larger, there are traces of fibrin, and of an oily matter, and the number of the globules is increased. In the chyle, after its exit from the mesenteric glands, the albumen, the fibrin, the oil, the globules, and more especially the two first and the last, are greatly increased. But in the chyle when it reaches the thoracic duct, these principles are so augmented, concentrated, and approximated to the state in which they exist in the blood, that the chyle is now capable of undergoing the characteristic process of the blood; for as the blood, when drawn from a vein, undergoes spontaneous coagulation, so the chyle, when drawn from the thoracic duct, separates into three parts; a solid substance or clot, which remains at the bottom of the vessel; a fluid which surrounds the clot; and a thin layer of matter, which is spread over the surface of the fluid. The solid substance is analogous to the fibrin, and the fluid to the serum of the blood; while the layer of matter which is spread over thefluid is of an oily nature: moreover, the chyle, when in contact with the air, quickly changes to a red colour, and abounds with minute particles of various sizes, but the largest of which is not yet equal to the diameter of the red particles of the blood.

698. The changes wrought upon the food, by which it is thus approximated to the chemical composition of the blood, are effected, as has been shown, partly by the gastric and intestinal juices, and partly by matters combined with the food highly animalized in their own nature, and endowed with assimilative properties, as the salivary secretion mixed with the food during mastication; the pancreatic and biliary secretions mixed with the food during the conversion of the chyme into chyle; and the mesenteric secretions mixed with the elaborated chyle of the mesenteric glands, and lastly, organized particles which have already formed a part of the living structures of the body mixed with the chyle under the form of lymph in the thoracic duct.

699. The lymph, until lately regarded as excrementitious, is really highly animalized, partly combined with the chyle as its last and highest assimilative matter; whence the compound formed by the admixture of chyle and lymph is far more proximate to the blood than the purest and most concentrated chyle; and partly returning with the chyle to the lungs, to receive there a second depuration, and thereby a higher elaboration.

700. There is evidence that there is a series of organs specially provided for the elaboration of the lymph no less than of the chyle. There are organs manifestly connected with the digestive apparatus, to which physiologists have found it extremely difficult to assign a specific office. These organs have a structure in some essential points alike; that structure is strikingly analogous to the organization of glands: like glands, they receive a prodigious quantity of arterial blood, and are supplied with a proportionate number of organic nerves; yet they are without an excretory duct. The organs in question are the bodies called the renal capsules, placed above the kidneys; the thyroid and thymus glands situated in the neck, and the spleen in close connexion with the stomach.

701. These organs, however analogous in structure to glands, cannot, it has been argued, be secreting organs, because they are destitute of an excretory duct, do not manifestly form from the blood any peculiar secretion, or, if they do, since there are no means of detecting where it is conveyed, it is impossible to understand how it is appropriated. But if these organs collect, concentrate, and elaborate lymph, preparatory to its admixture with the chyle and to its being sent a second time into the blood to undergo a second process of depuration, they perform the function of glands; and their want of an excretory duct, which has hitherto rendered their office so obscure, is accountedfor; they do not need distinct tubes for the transmission of any product of secretion; the lymphatic vessels which proceed from them and which convey the fluid they elaborate into the receptacle of the chyle, are their excretory ducts. That one of these organs, the spleen, is specially connected with the elaboration of the lymph, is manifest, both from its chemical nature and from the remarkable change which takes place in the chyle the moment the lymph from the spleen is mixed with it. Tiedemann and Gmelin state, as the uniform result of their observations and experiments, that the quantity of fibrin contained in the chyle is greatly increased, and that it actually acquires red particles as soon as the lymph from the spleen is mixed with it, and that the lymph from the spleen superabounds both with fibrin and with red particles. That the organs just enumerated, with the spleen, perform a similar function, is inferred from their being, like it, of a glandular structure, and without any excretory duct. If the spleen be really one of a circle of organs appropriated to a function such as is here supposed, a purpose is assigned to it adequate to its rank in the scale of organization; inferior to few, if its importance be estimated by the quantity of arterial blood with which it is supplied; yet this is the organ for which Paley could find no better use than that of serving for package.

702. But in whatever mode the lymph be elaborated, it is certain that it consists of matter highly animalized, and that its most important principles, its albumen, its fibrin, its globules, and even its salts, are in a chemical condition closely resembling that in which they exist in the blood.

703. It will appear hereafter that all the proximate principles of which the body is composed are reducible by analysis to three, namely, sugar, oil, and albumen: of these, sugar and oil are the least, and albumen the most highly organized. Every alimentary substance must contain at least one of these proximate principles, and in the various articles which compose an ordinary meal always two, and often all three, are afforded in abundance. From the phenomena which have been stated, it is clear that the digestive organs, in acting on these principles, exert the following powers.

1. A solvent power. The first action of the stomach on the alimentary substances presented to it is to reduce them to a fluid state. No substance is nutritious which is not a fluid, or capable of being reduced to a fluid. The stomach reduces alimentary substances to a fluid state by combining them with water. Water enters into the composition of organized bodies in two states, as an essential and as an accidental element. A quantity of water is contained in sugar when reduced to its dryest state; this water cannot be dissipated without the decomposition of the sugar;it is therefore an essential constituent of the compound. Water is combined with sugar in its moist state: of this water much may be removed without destroying the essential properties of the sugar: this part of the water is therefore said to be an accidental constituent of the sugar. In most cases organized bodies contain water in both these forms; and though it is commonly impossible to discriminate between the water that is essential and that which is accidental, yet the mode of union among the elements of bodies in these two states of their combination with water are essentially different. The stomach has the power of combining water with alimentary substances in both these forms. Thus fluid albumen, or white of egg, presented to the stomach is immediately coagulated or converted into a solid. Soon this solid begins to be softened, and the softening goes on until it is again reduced to a fluid. What was fluid albumen in the white of egg is now fluid albumen in chyme; but the albumen has undergone a remarkable change. Out of the stomach the albumen of the egg may be converted by heat into a firm solid; but the albumen of the chyme is capable of being converted only into a loose and tender solid. In passing from its state in the egg to its state in the chyme, the albumen has combined with a portion of water which has entered as an essential ingredient into its composition. By this combination the compound is reduced from what may be calleda strong to a weak state. This is the first action exerted by the stomach on most alimentary substances. They are changed from a concentrated to a diluted, from a strong to a weak state: the power by which the stomach effects this change is called its reducing power, and the agent by which it accomplishes it is the gastric juice; the essential ingredient of which has been shown to be muriatic acid, or chlorine (639,et seq.). The muriatic acid obtained from the common salt of the blood is poured in the form of gastric juice into the stomach, dissolves the food, combines it with water, reduces it from a concentrated solid to a dilute fluid; and thus brings it into the condition proper for the subsequent part of the process.

2. A converting power. Since whatever be the varieties of food, the chyme invariably forms a homogeneous fluid, the stomach must be endowed with the power of transforming the simple alimentary principles into one another; the saccharine into the oily, and the oily into the albuminous. The transformation of the saccharine into the oleaginous principle is traceable out of the body in the conversion of sugar into alcohol, which is essentially an oil. That the same transformation takes place within the body is indubitable. The oleagenous and the albuminous principles are already so nearly allied in nature to animal substance that they do not need to undergo any essential change in their composition.

3. A completing power. When the alimentary substances have been reduced and formed into chyme, when the chyme has been converted into chyle, and when the chyle absorbed by the lacteals is transmitted to the mesenteric glands, it undergoes during its passage through these organs a process the direct reverse of that to which it is subjected in the stomach; for whereas it is the office of the stomach to combine the alimentary substances with water, it is one office of the mesenteric glands to remove the superfluous water of the chyle; to abstract whatever particles of matter may be contained in the compound which are not indispensable to it, and to concentrate its essential constituents; and consequently these organs exert on the digested aliment a completing, in contradistinction to a reducing power.

4. A vitalizing power. When sugar is converted into oil, when oil is converted into albumen, when albumen, by the successive processes to which it is subjected is completed, that is, when the alimentary substances are made to approximate in the closest possible degree to the nature of animal substance, they must undergo a still further change, more wonderful than any of the preceding, and far more inscrutible; they must be endowed with vitality; must be changed from dead into living matter. Living substance only is capable of forming a constituent part of living substance. The ultimate action of thedigestive organs is the communication of life to the food, to which last and crowning process the reducing, converting, and completing processes are merely subordinate and preparatory. Of the agency by which this process is effected we are wholly ignorant; we know that it goes on; but the mode in which it is accomplished is veiled in inscrutable darkness.

704. Blood is alive; blood is formed from the food; life is communicated to the food before it is mixed with the blood. The blood is essentially albumen, which it contains in the form of albumen properly so called, in that of fibrin, and in that of red particles. In the thoracic duct the strong albumen of the lymph is mixed with the weaker albumen of the chyle. At the point where the thoracic duct terminates in the venous system, lymph and chyle are mixed with venous blood, and all commingled are borne directly to the lungs. There the carbon with which the venous blood is loaded is expelled in the form of carbonic acid gas; the particles of the lymph undergo some, as yet, unknown change, exalting their organization; and the water hitherto held in chemical union with the weak albumen of the chyle, is separated and carried out of the system together with the carbonic acid gas in the form of aqueous vapour. By this removal of its aqueous particles the ultimate completion is given to the digested aliment; and the weak and delicate albumen of the chyle is converted into the strong and firm albumen of the blood.

705. It has been stated (539), that though gelatin enters abundantly into the composition of many tissues of the body, and performs most important uses in the economy, it is never found in the blood; that it is formed from the albumen of the blood by a reducing process, in consequence of which carbon is evolved, which unites with the free oxygen of the blood, forming carbonic acid, thus conducing, among other purposes, to the production of animal heat. It is equally remarkable, that though the lymphatics or absorbents arise in countless numbers from every tissue of the body, and are endowed with the power of taking up every constituent particle of every organ, solid as well as fluid, yet gelatin is never found in the lymphatic vessels. The lymphatics contain only albumen in a form far more proximate to the blood than that of the chyle; consequently, before the gelatin of the body is taken up by the lymphatics, it must be reconverted into albumen; that is, the absorbed gelatin must undergo a process analogous to that which gelatin and other matters undergo in the stomach and duodenum; it follows that the digestive process is not confined to the stomach and duodenum, but is carried on at every point of the body. Hence there are two processes of digestion, a crude and a refined process. The crude process is carried on in the stomach and duodenum, in which dead animal matter is converted into living substance, as yet, however, possessing only the lowest kind of vitality.The capillary arteries receiving the substance thus prepared for them, build it up into structure perhaps the lowest and coarsest, the least organized, and capable of performing only the inferior functions.

706. Capillary arteries in countless numbers terminate in the tissues in membraneless canals (304 and 310). Particles of the blood are seen to quit the arterial stream and to enter into the tissues, becoming a component part of them: other particles are seen to quit the tissues and to enter the current of the blood. The latter are probably organic particles, to which a certain degree of elaboration has been already given, now transmitted to the capillary veins, to be carried back to the lungs to undergo there a further depuration, fitting them on their return to the system for a higher organization.

707. Thus the lymphatic vessels, analogous in so many other respects to the veins, are probably similar to them in this also—that they take up from the tissues particles already organized, in order to submit them to processes which communicate to them a progressively higher organization. The notion that the contents of the lymphatics consist of worn-out particles, capable of accomplishing no further purpose in the economy, is not tenable:—

1. Because it is not analogous to the ordinary operations of nature to mix wholly excrementitious matter with a substance for the production, elaboration, and perfection of which, she has constructed such an expensive apparatus.

2. Because, on the other hand, the admixture of matter already highly animalized with matter, as yet but imperfectly animalized, exalts the nature of the latter, and is conducive to its complete animalization.

3. Because the lymph, almost wholly albuminous, is already closely allied in nature to the blood; it is, therefore, reasonable to infer, that it is matter passing through an advancing stage of purification and exaltation.

4. Because this plan of progressive organization is in harmony with the ordinary operations of nature, in which there is traceable a successive ascent from the low to the high, the former being preparatory and necessary to the latter. The tender and delicate organs of animal life, the brain, the nerves, the apparatus of sense, the muscles, inasmuch as they perform the highest functions, probably require to be constructed of a more highly organized material, for the production of which the matter primarily derived from crude aliment is subjected to different processes, rising one above the other in delicacy and refinement; by each of which it is made successively more and more perfect, until it acquires the highest qualities of living substance, and is capable of becoming the instrument of performing its most exalted functions.

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