cellular passages between the liver-cells. This part of the subject will be more fully discussed in Chapter V.

GALL-BLADDER.

The gall-bladder may be looked upon as a diverticulum of the hepatic duct. It lies in a fossa underneath the liver. It is of a pear shape, and its fundus is directed downwards and forwards; it terminates in the cystic duct, which is about an inch in length. The hepatic duct, formed by the union of the right and left ducts, leaving the liver by the transverse fissure, passes downwards, and soon joins the cystic duct at an acute angle, to form the ductus communis choledochus, which is about three inches in length, and lies between the layers of the gastro-hepatic omentum. After coming into close proximity with the pancreatic duct, the common duct enters the coats of the intestine with the latter, and passes obliquely between them for three-quarters of an inch. The ducts open by an orifice common to both at the junction of the descending and transverse portions of the duodenum.

The mucous membrane of the gall-bladder is thrown into reticulated folds, which form the boundaries of numerous polygonal depressions, so that upon its internal surface it presents a honeycombed appearance. It is highly vascular, and is covered with columnar epithelium. The folds are prolonged into the cystic duct, where they are arranged in a crescentic manner, their general direction being that of a spiral, and they have been compared to a spiral valve. The peculiar arrangement of the vessels of the gallbladder has already been described in page 26.* The cystic artery is derived from the right division of the hepatic, and the veins empty themselves into the vena porta. The lymphatics are very numerous. The greater part of the thickness of the walls of the gall-bladder is composed of fibrous tissue, but there also exists a layer, in which muscular fibre-cells, taking partly a longitudinal and partly a transverse direction, are found. The human gallbladder is capable, under ordinary circumstances, of containing about one ounce of fluid; but it undergoes great alterations in

* Through the kindness of his friend, Professor Weber, the author has just had an opportunity of reading his researches upon the anatomy of the liver, in which he refers to the distribution of the vessels upon the gall-bladder, described in p. 26. Weber is the only author who notices this very beautiful and unusual arrangement of the vessels.

volume, and in it the bile becomes inspissated, and undergoes other changes. Dr. Kemp has shown, in some recent experiments, that the mucus of the gall-bladder takes a very active part in altering the character of hepatic bile and converting into cystic bile; changes which consist not merely in its inspissation but in an alteration in its chemical composition.*

The gall-bladder is absent in many fishes; in pigeons, toucans, and some other birds; in the elephant, stag, horse, and tapir; but it is present in the ox, sheep, and antelope. It is always found among reptiles. The reason of its absence in the animals above alluded to has not yet been satisfactorily explained.

Nerves and Lymphatics.-The nerves of the liver are branches of the sympathetic, which are principally distributed upon the artery, and the mode of their arrangement is very similar to that of other arterial trunks. I have not been able to demonstrate them on small branches less than the 1-500th of an inch in diameter.

The lymphatics are abundant in the portal canals and in the transverse fissure, but I have never been able to trace them to their origin. The manner in which the lymphatics may be injected has been described in page 5. If any vessels of the liver be injected with water or size the lymphatics are always distended. I have not been able to demonstrate their presence in portal canals less than the quarter of an inch in diameter.

HEPATIC VEIN.

The radicle of the hepatic vein (intralobular vein) is always seen at a point midway between branches of the portal vein, or in the centre of the lobule in the case of the pig. The capillary meshes are elongated as they approach the hepatic vein, and they converge towards this vessel, the smallest trunk of which is larger than the smallest branch of the portal vein.

If the trunk of an hepatic vein of moderate size be laid open, numerous small openings will be observed upon its interior. These openings correspond to the centres of the lobules which surround the trunk of the vein, as described by Kiernan. It is by reason of the entrance of these minute intralobular branches into the large trunk of the vein, that the latter remains patent when cut across. The small radicles of the hepatic vein receive capillaries on all * Proceedings of the Royal Society, Vol. VIII., No. 21, p. 133. 1856.

sides, quite down to the point where they emerge from the lobule; and these small intralobular veins, in many instances, open, not into a vessel somewhat larger than themselves, but pour their blood at once into a large trunk. In these points the arrangement of the portal trunks contrasts remarkably with that of the branches of the hepatic vein.

The reason of this difference is clear. In no other manner, consistent with the compact nature of the mammalian liver, and without loss of space, could a part of the portal blood be equally distributed to, and made to pass through, the extensive capillary system existing in the lobules. After having passed through this plexus, the blood is poured into the veins, and carried away from the liver in the most direct manner possible.

Hepatic Vein in the Seal.-In the seal, the capacity of the branches of the hepatic vein within the liver is enormously greater than in other animals, an arrangement which has reference to the accumulation of blood in this organ, and in the great venous sinus into which the trunk of the hepatic vein is dilated, when the animal remains for some time under water. Accumulation of the blood, in the small branches, to an improper extent, is prevented, while its further progress towards the hepatic venous sinus, after its accumulation, is promoted by a beautiful arrangement to which I must briefly refer.

This peculiarity was first noticed by Mr. Kiernan, who describes the external coat as consisting of circular fibres. In the smaller trunks these fibres are arranged in the form of circular fasciculi, external to which is a certain quantity of lax areolar tissue, which permits of great alteration in the volume of the veins taking place.* When the hepatic veins were injected with plain size, I was surprised to find that a beaded appearance was produced, which is represented in fig. 36, in which this arrangement is very well shown. Upon making a section of a small branch of the vein which had been injected, it was seen to be partially divided into a number of small chambers, by means of septa, which are shown in the figure. At the situation of each partition a circular band of muscular fibre-cells, which is enclosed at intervals in the lax fibrous coat, may be very easily demonstrated. It is no doubt the office of p. 738.

Op. Cit.

these contractile circular partitions to prevent undue distension, to equalize the accumulation of blood in the branches of the vein, to drive it onwards towards the cava, and to prevent undue pressure upon the secreting structure of the lobule, which is separated from the vein when it is distended, by the slight distance represented in fig. 36 at the lower part of the drawing. The arrangement of these fibres is shown in the same figure, and the circular muscular bands are represented as divided transversely. Occasionally an anastomosis takes place between the small branches of the hepatic vein. One of the most remarkable is represented in fig. 6, which is copied from a preparation taken from the human liver.

39

CHAPTER IV.

OF THE INTIMATE STRUCTURE OF THE LOBULES OF THE LIVER. -EVIDENCE OF THE EXISTENCE OF A TUBULAR BASEMENT MEMBRANE WITHIN WHICH THE LIVER-CELLS ARE CONTAINED. DISTENSION OF THE TUBES BY INJECTION. RADIATION OF THE TUBES FROM THE CENTRE OF THE LOBULE.-DIAMETER OF THE TUBES OF THE NETWORK.INTERVAL BETWEEN THE WALLS OF THE CAPILLARIES, AND THE MEMBRANE OF THE TUBULAR NETWORK.-CONTENTS OF THE TUBULAR NETWORK OF BASEMENT MEMBRANE.-CELLS.-CONTENTS OF THE CELL.-TUBULAR NETWORK IN FISHES. OF THE MANNER IN WHICH THE CELLS ARE ARRANGED WITHIN THE TUBULAR MEMBRANE.

A LOBULE of the liver consists of a solid network of capillary vessels, in the meshes of which the liver-cells are seen. Towards the central part of the lobule, the cells assume a more linear direction, and appear to radiate from the intralobular vein as from a centre. It has long been a question whether the cells simply lie in the meshes of the capillary network, or whether they are enclosed in tubes of basement membrane. The cells are prone to become detached in the form of little columns which are often branched, an appearance which is usually considered to be due to the manner in which they are fitted into the vascular meshes, and not to the presence of any tissue surrounding them, which would keep them together in this manner. In these detached masses it is not easy to demonstrate a basement membrane, and their connexion has been accounted for by a supposed adhesion of the cells to each other.

Among the authorities who have advocated the existence of a basement membrane may be mentioned Krukenberg, Schröder Van der Kolk, Retzius, Weber, Theile, Backer, and Leidy, while Kiernan described a lobular biliary plexus, the tubes of which were considered by him to be continuous with the ducts, nearly five