The precise use of the pancreatic juice in digestion is not determined.

4. Secretion of the Bile.

The biliary secretion is, also, a digestive fluid, of which we have spoken in the appropriate place. The mode, however, in which the process is effected, has not yet been investigated.

Fig. 122.

The apparatus consists of the liver, which accomplishes the formation of the fluid; the hepatic duct, the excretory channel, by which the bile is discharged; the gall-bladder, in which a portion of the bile is retained for a time; the cystic duct—the excretory channel of the gall-bladder; and the ductus communis choledochus, or choledoch duct, formed by the union of the hepatic and cystic ducts, and which conveys the bile immediately into the duodenum. The liver, A, A, Fig. 92, and A, A, Fig. 122, is the largest gland in the body; situated in the abdomen, beneath the diaphragm, above the stomach, the arch of the colon, and the duodenum; filling the whole of the right hypochondrium, and more or less of the epigastrium, and fixed in its situation by duplicatures of the peritoneum, called ligaments of the liver.

The weight of the human liver is generally, in the adult, about three or four pounds. In disease, however, it sometimes weighs twenty or twenty-five pounds; and, at other times, not as many ounces. Its shape is irregular, and it is divided into three chief lobes, the right, the left, and the lobulus spigelii. Its upper convex surface touches every where the arch of the diaphragm. The lower concave surface corresponds to the stomach, colon, and right kidney.

At the concave surface, two A, A. Concave surface of liver turned upwards, and to the right side.-B. Lobulus spigelii. fissures are observable;—the one-Between B and C, the porta of the liver.-D. Ligamentum rotundum.-E, F. Gall-bladder.passing from before to behind, and G. The pancreas.-H. The spleen.-I. The lodging the umbilical vein in the ribs.-K, K. The kidneys.-L, L. Renal veins. fœtus-called the horizontal sulcus arteries.-Q, Q. Common iliac arteries.-R. Veor fissure, great fissure, or fossa umbilicalis; the other, cutting the bladder. last at right angles, and running from right to left, by which the

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na cava.-S. The spermatic vein.-U, U. Common iliac veins.-V. End of colon.-X. Commencement of the rectum.-Y, y. Urinary

different nerves and vessels proceed to and from the liver, and called the principal fissure, or sulcus transversus.

The liver itself is composed of the following anatomical elements: -1. The hepatic artery, a branch of the coeliac, which ramifies minutely through the substance of the organ. The minuter branches of this artery are arranged somewhat like the hairs in a painter's brush, and have hence been called the penicilli of the liver. 2. The vena porta, which we have elsewhere seen to be the common trunk of all the veins of the digestive organs and of the spleen. It divides like an artery, its branches accompanying those of the hepatic artery. Where the vein lies in the transverse fissure, it is of great size, and has hence been called sinus venæ portæ. The possession of two vascular systems, containing blood, is peculiar to the liver, and has been the cause of some difference of opinion, with regard to the precise material—arterial or venous—from which the bile is derived. 3. The excretory ducts or biliary ducts. These are presumed to arise from acini, communicating, according to some, with the extremities of the vena portæ; according to others, with the radicles of the hepatic artery. At their commencement, they are termed pori biliarii. These ultimately form two or three large trunks, which issue from the liver by the transverse fissure, and end in the hepatic duct. 4. Lymphatic vessels. 5. Nerves, in small number, compared with the size of the liver, some proceeding from the eighth pair; but the majority from the solar plexus, and following the course and divisions of the hepatic artery. 6. The supra-hepatic veins, or venæ cavæ hepaticæ, which arise in the liver by imperceptible radicles, communicating with the final ramifications of both the hepatic artery and vena portæ. They return the superfluous blood, carried to the liver by these vessels, by means of two or three trunks, and six or seven branches, which open into the vena cava inferior. These veins generally pass, in a convergent manner, towards the posterior margin of the liver, and cross the divisions of the vena portæ at right angles. 7. The remains of the umbilical vein, which, in the fœtus, enters at the horizontal fissure. This vein, after respiration is established, becomes converted into a ligamentous substance, called, from its shape, ligamentum rotundum, or round ligament. The parenchyma, formed by these anatomical elements, it is difficult to describe; and although the term liver-coloured is used in common parlance, it is not easy to say what are the ideas attached to it.

The organ has two coats;—the outer, derived from the peritoneum, which is very thin, transparent, easily lacerable, and vascular, and is the seat of the secretion, operated by serous membranes in general. It does not cover the posterior part, nor the excavation for the gall-bladder, the vena cava, nor the fissures in the concave surface of the liver. The inner coat is the proper membrane of the liver. It is thin, but not easily torn, and it covers not only every part of the surface of the liver, but also the large vessels

that are proper to the organ. The condensed cellular substance,which unites the sinus of the vena portæ and its two great branches, the hepatic artery, the common biliary duct, lymphatic glands, lymphatic vessels, and nerves in the transverse fossa or fissure of the liver, was described by Glisson as a capsule; and hence has been called the capsule of Glisson.

The gall-bladder, (Figs. 92, 121, and 122,) is a small membranous pouch, of a pyriform shape, situated at the inferior and concave surface of the liver, to which it is attached, and above the colon and duodenum. A quantity of bile is usually found in it.

The gall-bladder is not found in all animals. It is wanting in the elephant, horse, stag, camel, rhinoceros, and goat; in certain of the cetacea; and in some birds, as the ostrich, pigeon, and parrot; and is occasionally deficient in man.

Its largest part or fundus, Figs. 121 and 122, is turned forwards; and, when filled, frequently projects beyond the anterior margin of the liver. Its narrowest portion, cervix or neck is turned backwards, and terminates in the cystic duct. Externally, it is partly covered by the peritoneum which attaches it to the liver, and to which it is, moreover, adherent by cellular tissue and vessels.

Internally, it is rugous; the folds being reticulated, and appearing somewhat like the cells of a honey-comb.

Anatomists have differed with regard to the number of coats proper to the gall-bladder. Some have described two only;--the peritoneal and mucous; others have added an intermediate cellular coat; whilst others have reckoned four;—a peritoneal coat;—a thin stratum of muscular fibres, passing in different directions, and of a pale colour; a cellular coat, in which a number of blood-vessels is situated; and an internal mucous coat. The existence of the muscular coat has been denied by perhaps the generality of anatomists; but there is reason for believing in its existence. Amussat saw muscular fibres distinctly in a gall-bladder dilated by calculi; and Dr. Monro, the present Professor of anatomy in the University of Edinburgh, asserts, that he has seen it contract, in a living animal, for half an hour, under mechanical irritation, and assume the shape of an hour-glass.

The mucous coat forms the rugæ to which we have already alluded. In the neck, and in the beginning of the cystic duct, there are from three to seven,—sometimes twelve,—semilunar duplicatures, which retard the flow of any fluid inwards or outwards. These are sometimes arranged spirally, so as to form a kind of valve, according to Amussat.

On the inner surface of the gall-bladder, especially near its neck, numerous follicles exist; the secretion from which is said to fill the gall-bladder, when that of the bile has been interrupted by disease, as in yellow fever, scirrhus of the liver, &c.

The hepatic duct, Fig. 121, a, is the common trunk of all the excretory vessels of the liver; and makes its exit from that organ

by the transverse fissure. It is an inch and a half in length, and about the diameter of an ordinary writing quill. It is joined, at a very acute angle, by the duct from the gall-bladder—the cystic duct, Fig. 121, e, to form the ductus communis choledochus. The cystic duct is about the same length as the hepatic.

The ductus communis choledochus is about three or three and a half inches long. It descends behind the right extremity of the pancreas, through its substance; passes for an inch obliquely between the coats of the duodenum, diminishing in diameter; and ultimately terminates by a yet more contracted orifice, on the inner surface of the intestine, at the distance of three or four inches from the stomach.

The structure of all these ducts is the same. The external coat is thick, dense, strong, and generally supposed to be of a cellular character; the inner is a mucous membrane, like that, which lines the gall-bladder.

The secretion of the bile is probably effected like the other glandular secretions; modified, of course, by the peculiar structure of the liver. We have seen that the organ differs from every other secretory apparatus, in having two kinds of blood distributed to it:—arterial blood by the hepatic artery; and venous blood by the vena portæ. A question has consequently arisen—from which of these is the bile formed?

Anatomical inspection throws no light on the subject; and, accordingly, argument is all that can be adduced on one side or the other.

The most common and the oldest opinion is, that the bile is separated from the blood of the vena portæ; and the chief reasons, adduced in favour of this belief, are the following. First. The blood of the portal system is better adapted than arterial blood for the formation of bile, on account of its having, like all venous blood, more carbon and hydrogen, which are necessary for the production of a humour as fat and oily as the bile; and it has been imagined, by some, that the blood, in crossing the omentum, becomes loaded with fat. Secondly. The vena portæ ramifies in the liver, after the manner of an artery, and evidently communicates with the secretory vessels of the bile. Thirdly. It is larger than the hepatic artery; and more in proportion to the size of the liver; the hepatic artery seeming to be merely for the nutrition of the liver, as the bronchial artery is for that of the lung.

In answer to these positions it has been argued; that there seems to be no more reason why the bile should be formed from venous blood than the other fatty and oleaginous humours—the marrow and fat, for example,—which are derived from arterial blood.

It is asked, again, whether, in fact, the blood of the vena porta is really more rich in carbon and hydrogen? and whether there is a closer chymical relation between the bile and the blood of the vena

portæ, than between the fat and arterial blood? The notion of the absorption of fat from the omentum, it is properly urged, is totally gratuitous. Secondly. Admitting that the vena portæ is distributed to the liver after the manner of an artery; is it clear, it has been asked, that it is inservient to the biliary secretion? Thirdly. If the vena portæ be more in proportion to the size of the liver than the hepatic artery, the latter appears to bear a better ratio to the quantity of bile secreted; and, moreover, it is probable, as has been shown in another place, that the liver has other functions connected with the portal system, in the admixture of heterogeneous liquids absorbed from the intestinal canal.

In the absence of direct experiment, however, physiologists have usually embraced one or other of these exclusive views. The generality, as we have remarked, assign the function to the vena portæ. Bichat, on the other hand, ascribes it to the hepatic artery. Broussais thinks it probable, that the blood of the vena portæ is not foreign to the formation of the bile, since it is confounded with that of the hepatic artery in the parenchyma of the liver; "but to say with the older writers, that the bile cannot be formed but by venous blood, is, in our opinion" he remarks "to advance too bold a position, since the hepatic artery sends branches to each of the glandular acini, that compose the liver." Magendie likewise concludes, that nothing militates against the idea of both kinds of blood serving in the secretion; and that it is supported by anatomy; as injections prove, that all the vessels of the liver,—arterial, venous, lymphatic, and excretory,—communicate with each other.

The view, that ascribes the bile to the hepatic artery, appears to us the most probable. It has all analogy in its favour. We have no disputed origin as regards the other secretions. They all proceed from arterial blood; and function sufficient, we think, can be assigned to the portal system, without conceiving it to be concerned in the formation of bile. We have, moreover, pathological cases, which prove that the bile can be formed from the blood of the hepatic artery. Mr. Abernethy met with an instance, in which the trunk of the vena portæ terminated in the vena cava; yet bile was found in the biliary ducts, which could have been derived only from the hepatic artery. A similar case is given by Mr. Lawrence; and the present Professor Monro, in his "Elements of Anatomy" details a case communicated to him by the late Mr. Wilson, of the Windmill street school, in which there was reason to suppose, that the greater part of the bile had been derived from the hepatic artery. The patient, a female, thirteen years old, died from the effects of an injury of the head. On dissection, Mr. Wilson found a large swelling at the root of the mesentery, consisting of several absorbent glands in a scrofulous state. Upon cutting into the mass, he accidentally observed a large vein passing directly from it into the vena cava inferior, which, on dissection, proved to be the vena portæ; and on tracing the vessels entering into it, one was found to