from the intestinal contents; (3) migration of the lymph-corpuscles carrying the incepted fat-particles by their amoeboid movements through the tissue of the villus and into the central lacteal; (4) disintegration and solution of the immigrated lymph-corpuscles, and setting free both of their fatty contents and also of the proteid matters of which they are themselves composed.

FIG. 185, A.-SECTION OF THE VILLUS OF A RAT KILLED DURING FAT-ABSORPTION. ep, epithelium; str, striated border; c, lymph-cells; c', lymph-cells in the epithelium; 1, central lacteal containing disintegrating lymph-corpuscles.

FIG. 185, B.-MUCOUS MEMBRANE OF FROG'S INTESTINE DURING FAT-ABSORPTION. ep, epithelium; str, striated border; c, lymph-corpuscles; 7, lacteal.

This migration of the lymph-corpuscles into the lacteals of the villi is not a special feature of fat-absorption alone, but occurs even when absorption of other matters is proceeding; so that the transference of fat-particles is merely a part of a more general phenomenon accompanying absorption.

THE LARGE INTESTINE.

The large intestine has the usual four coats, except near its termination, where the serous coat is absent. The muscular coat is peculiar in the fact that along the cæcum and colon the longitudinal muscular fibres are gathered up into three thickened bands which produce puckerings in the wall of the gut.

str

The mucous membrane of the large intestine is beset with simple tubular glands somewhat resembling the crypts of Lieberkühn of the small intestine, and lined by columnar epithelium similar to that of the inner surface of the gut, but containing many more mucussecreting or goblet cells (fig. 186). The extremity of each gland is usually slightly dilated. The interglandular tissue is like that of the

stomach, as is also the arrangement of the blood-vessels and lymphatics in it. The nerves of the large intestine also resemble those of the small intestine and stomach in their arrangement.

At the lower end of the rectum the circular muscular fibres of the gut become thickened a little above the anus so as to form the internal sphincter muscle. In this region also there are a number of compound racemose mucous glands opening on to the surface of the mucous membrane.

LESSON XXXI.

STRUCTURE OF THE LIVER AND PANCREAS.

1. MAKE sections of liver and study them carefully with a low and high power. Sketch the general arrangement of the cells in a lobule under the low power and under the high power. Make very careful drawings of some of the hepatic cells and also of a portal canal.

2. Study, first of all with the low and afterwards with a high power, a section of the liver in which both the blood-vessels and the bile-ducts have been injected. Make a general sketch of a lobule under the low power and draw a small part of the network of bile-canaliculi under the high power.

3. Tease a piece of fresh liver in salt solution for the study of the appearance of the hepatic cells in the recent living condition.

4. Prepare sections of the pancreas from a gland which has been hardened in alcohol. The sections are stained with borax-carmine and mounted in the usual way in Canada balsam.

Make a sketch under the low power.

5. Tease a small piece of fresh pancreas in salt solution. Notice the granules in the alveolar cells, chiefly accumulated in the half of the cell which is nearest the lumen of the alveolus, leaving the outer zone of the cell clear. Sketch a small portion of an alveolus under a high power.

THE LIVER.

The liver is a solid glandular mass, made up of the hepatic lobules. These are polyhedral masses (about 1 mm. in diameter) of cells, separated from one another by connective tissue. In some animals, as in the pig, this separation is complete, and each lobule is isolated, but in man it is incomplete. There is also a layer of connective tissue underneath the serous covering of the liver, and forming the so-called capsule of the organ.

The blood-vessels of the liver (portal vein and hepatic artery) enter it on its under surface, where also the bile-duct passes away from the gland. The branches of these three vessels accompany one another in their course through the organ, and are enclosed by loose connective tissue (capsule of Glisson), in which are lymphatic vessels, the whole being termed a portal canal (fig. 187). The smallest branches of the vessels penetrate to the intervals between the hepatic lobules, and are known as the interlobular branches. The blood leaves the liver at the back of the organ by the hepatic veins: the branches of these run through the gland unaccompanied by other vessels (except

lymphatics) and can also be traced to the lobules, from each of which they receive a minute branch (intralobular vein) which passes from the centre of the lobule, and opens directly into the (sublobular) branch of the hepatic vein.

d.

FIG. 187.-SECTION OF A PORTAL CANAL.

a, branch of hepatic artery; v, branch of portal vein; d, bile-duct; 7,7, lymphatics in the areolar tissue of Glisson's capsule which encloses the vessels.

FIG. 188.-DIAGRAMMATIC REPRESENTATION OF TWO HEPATIC LOBULES. The left-hand lobule is represented with the intralobular vein cut across; in the right-hand one the section takes the course of the intralobular vein. p, interlobular branches of the portal vein; h intralobular branches of the hepatic veins; s, sublobular vein; c, capillaries of the lobules. The arrows indicate the direction of the course of the blood. The liver-cells are only represented in one part of each lobule.

Each lobule is a mass of hepatic cells pierced everywhere with a network of blood-capillaries (fig. 188), which arise at the periphery of the lobule, there receiving blood from the interlobular branches of the portal vein (p), and converge to the centre of the lobule, where they unite to form the intralobular branch of the hepatic vein. The inter

lobular branches of the hepatic arteries join this capillary network a short distance from the periphery of the lobule.

The hepatic cells (fig. 189), which everywhere lie between and surround the capillaries, are polyhedral, somewhat granular-looking cells,

FIG. 189.-SECTION OF RABBIT'S LIVER WITH THE INTERCELLULAR NETWORK OF BILE-CANALICULI INJECTED. (Highly magnified.) (Hering.)

Two or three layers of cells are represented; b, b, blood-capillaries.

each containing a spherical nucleus.

After a meal, the cells in the outer part of the lobule may become filled with fat, and masses of glycogen can also frequently be seen within the cells.

The bile-ducts commence between the hepatic cells in the form of fine canaliculi, which lie between the adjacent sides of two cells, and form a close network, the meshes of which correspond in size to the cells (fig. 189). At the periphery of the lobule these fine canaliculi pass into the interlobular bile-ducts (fig. 190), the columnar epithelium-cells of which become, by a gradual transition, changed into cubical and polyhedral cells, which join those of the hepatic lobules.

The bile-ducts are lined by clear columnar epithelium (fig. 187, d). Outside this is a basement-membrane, and in the larger ducts some fibrous and plain muscular tissue. Many of the larger ducts are beset with small cæcal diverticula.

The gall-bladder is in its general structure similar to the larger bileducts. It is lined by columnar epithelium, and its wall is formed of fibrous and muscular tissue.

The lymphatics of the liver are said to commence as perivascular lymphatic spaces enclosing the capillaries of the lobules. Efferent lymphatics pass away from the organ in the connective tissue which invests the portal and hepatic veins.