tions. They are smaller than the hepatic cells, and contain one or two nuclei.

In a recent communication Kupffer (99) states that the stellate cells belong to the endothelium of the intralobular capillaries of the portal vein. In such cells blood-corpuscles and fragments of such were often found. The endothelium of these capillaries possesses, therefore, a phagocytic function, taking up particles of foreign matter, blood-corpuscles, etc.

The efferent ducts of the liver, the bile-ducts, are lined by columnar epithelium, varying in height in direct proportion to the caliber of the passage. The smallest ducts possess a low, the medium. sized a cubical, and the larger a columnar epithelium. The smaller bile-ducts have no clearly defined external walls other than the membrana propria; the larger ones, on the other hand, possess a

Fig. 224.-Connective tissue from liver of sturgeon. At a is an open space from which the hepatic cells were mechanically removed during treatment.

connective-tissue sheath which may even present two layers in the larger passages. Unstriped muscular fibers occur in the large ducts, but do not forma continuous layer until the gall-bladder is reached, where two layers are found. The epithelium of the gall-bladder is of the columnar variety, with nuclei in the lower thirds of the cells; a cuticular zone is either absent or very poorly developed. The mucous membrane of the gall-bladder is raised into folds having a peculiar reticular arrangement. The gall-bladder contains a few mucous glands; these are, however, more numerous in the hepatic, cystic, and common bile-ducts.

Besides the network of lymph-vessels accompanying the portal vein and hepatic artery, there are also lymphatic networks about the branches of the hepatic vein (v. Wittich). The lymph-vessels penetrate the liver lobules and pass between the hepatic cells

and the blood capillaries to form perivascular capillary lymph

spaces.

Berkley (94) has described several divisions of the intrinsic nerves of the liver, all connected and morphologically alike. These nerves are no doubt the neuraxes of sympathetic neurones, the cell-bodies of which are located in ganglia outside of this organ. No medullated fibers were found by him, although it seems probable that the nerve-fibrils terminating between the cells of the bile-ducts (see below) are terminal branches of sensory nerve-fibers. The nerves of the liver accompany the portal vessels, the hepatic arteries, and the bile-ducts. The first division of the nerves, embracing the larger number of the intrinsic hepatic nerves, accompany the branches of the portal vessels, form plexuses about them, and end in interlobular and intralobular ramifications, the latter showing here and there knob-like terminations on the liver-cells, and, in their course, give off here and there branches which end on the portal vessels.

Fig. 225.-Part of a section through liver lobule from dog, showing stellate cells;

X168 (vid. T. 257).

The nerve-fibers following the hepatic arteries are in every respect like the vascular nerves in other glands.

Some of the terminal branches seem, however, to end on hepatic cells. The nerve-fibers following the bile-ducts may be traced to the smaller and medium-sized ducts, forming a network about them, and ending here and there in small twigs on the outer surface of the cells, and occasionally, it would seem, between the epithelial cells lining the ducts. The suggestion seems warranted that these terminal fibrils are the endings of sensory nerves. Some of the nerve-fibers following the bile-ducts may be traced into the hepatic lobules. The intralobular plexus is formed, therefore, by the terminal branches of the nonmedullated nerve-fibers accompanying the portal and hepatic vessels and the bile-ducts. In the wall of the gall-bladder are found numerous small sympathetic ganglia formed by the grouping of the cell-bodies of sympathetic neurones (Dogiel). The neuraxes of these

innervate the nonstriated muscle of this structure. Large, medullated nerve-fibers may be traced through these ganglia which appear to end in free sensory endings in and under the epithelium lining the gall-bladder (Huber).

In the human embryo the liver originates from the intestine during the second month as a double ventral diverticulum. Later solid trabecular masses are developed which then unite and become hollow. At this stage the whole gland is uniform in structure, as a division into lobules does not take place until later. The bile capillaries are surrounded by more than two rows of cells. In this stage the embryonal liver suggests a condition which is permanent during the life of certain animals. Only later when the venæ advehentes, which later represent the branches of the portal vein, penetrate the liver, is there a secondary division into lobules (about the fourth month), by which process the primitive type gradually changes to that characteristic of the adult.

E. THE PANCREAS.

Like the liver, the pancreas is an accessory intestinal gland, and originates as a diverticulum of the intestine. It remains in permanent communication with the intestine by means of its duct-the pancreatic or Wirsungian duct. The pancreas is composed of numerous microscopic lobules,

Nucleus and outer zone.

Inner granu

lar zone.

alveolus of frog's pancreas. Technic No. Fig. 226.-Transverse section through

125.

surrounded by connective tissue which penetrates into the lobules and between the alveoli and is accompanied by vessels and nerves. The secretory portion of the organ may be regarded as a branched tubulo-acinal gland with terminal alveoli, the latter forming the principal portion of the gland. The epithelial walls of the alveoli consist of a number of secretory cells, whose appearance varies according to the functional state. of the organ. The basilar portions of the cells present a homogeneous protoplasm, while those parts of the cells bordering upon the lumen are granular. The relation of these zones to each other depends upon the physiologic condition of the gland; during starvation the internal or granular zone is wide and prominent; after moderate secretion the cells become as a whole somewhat smaller, the granules decrease in number, and the outer or protoplasmic zone increases in size. After prolonged secretion there is an entire absence of the granules, and the whole cell apparently consists of homogeneous protoplasm// It is therefore probable

that during a state of rest peculiar granules (zymogen granules) are formed at the expense of the protoplasm, and that these granules represent a preliminary stage of the finished secretion. During the functional activity of the gland the granules gradually disappear, while the fluid secretion simultaneously makes its appearance in the lumen, although the granules have as yet never been observed in the lumen itself. After secretion the cell grows again until it reaches its original size, only again to begin the formation of zymogen granules. Whether the cells of the gland are destroyed or not during secretion is still a matter of uncertainty.

An intermediate tubule similar to those of the salivary glands connects with each alveolus, and then passes over into a short intralobular duct. This is lined, as in the salivary glands, with columnar epithelial cells, which are not, however (at least in man), striated at their basal ends. The intralobular ducts merge

Fig. 227. From section through human pancreas; X about 200 (sublimate). into excretory ducts, which finally empty into the pancreatic duct. The epithelium of the excretory ducts is simple columnar in type. Goblet cells are seen only in the pancreatic duct.

In the secreting alveoli small protoplasmic, polygonal, and even stellate cells are often seen, the so-called centro-acinal cells, or cells of Langerhans. The significance of these structures is not fully understood. Langerhans himself supposed that they belonged to the walls of the excretory ducts. This interpretation seems warranted by the fact that it has been found that the secreting cells of the alveoli are directly joined to the low cells of the intermediate tubules. When the alveoli lie closely packed together, the adjoining intermediate tubules fuse and are reduced to one or, at most, a few cells. As a result a condition is seen within the alveolar complexus, especially when the excretory ducts are in a collapsed state, closely resembling the structures seen by Langer

hans. Peculiar cells, wedged in here and there between the secretory cells, but resting on the membrana propria, have also been observed. They undoubtedly are sustentacular cells of the gland (cuneate cells, Podwyssotzki, 82).

The membrana propria of the alveoli is probably homogenous. Immediately adjoining it is another delicate but firm membrane, consisting of fibrils whose structure in many respects resembles that of the reticular fibers (Gitterfasern) in the liver and spleen, but which are here in relation to the alveoli (Podwyssotzki, 82).

In warm- and cold-blooded animals, groups of cells differing in arrangement, size, and structure from the secretory cells, are found among the gland tubules and alveoli of the pancreas; these are known as the intertubular cell-masses, or areas of Langerhans. They

Fig. 228. From section through human pancreas; X 450 (sublimate).

consist of slightly granular cells, smaller than the secretory cells of the alveoli, arranged in the form of anastomosing trabecula, with irregular spaces, varying in size, separating the trabeculæ. Dogiel (93) has shown that in a well-preserved human pancreas treated by the chrome-silver method, in which the gland ducts even to their finest intra-alveolar branches were well stained, no ducts were found in the areas of Langerhans. Such areas are, in the human pancreas, usually separated from the surrounding gland tissue by a small amount of connective tissue. They possess a blood supply, consisting of relatively large capillaries found in the spaces formed by the trabeculæ of cells above mentioned. The areas of Langerhans have been variously interpreted. They have been looked upon as small areas of gland tissue in process of degeneration, or