OF THE PANCREAS. THIS gland resembles the salivary glands in structure, being composed of small glandular vesicles, which contain fat-globules, and are lined by epithelium cells. The elements of the gland are united together by connective tissue, in which the vessels, nerves, and ducts are distributed. For microscopic examination the pancreas of a small animal, as a mouse, rat, or rabbit, is to be selected, and a small portion, if possible a single lobule, cut off with fine scissors, and placed on the slide in a drop of water or serum; the covering glass is to be applied without pressure, and the specimen immediately examined, first under the low, and afterwards under a high power. If the pancreas of a large animal be employed, it will be necessary to slightly teaze out a small portion, or to press it flat with the covering glass, before putting it under the microscope. The illustration shows the arrangement of the small vesicles in one of the lobules of the pancreas of a mouse. Minute lobule or acinus of the pancreas of a mouse, showing the two forms or stages of the epithelium, and the varied forms and sizes of the The pancreatic fluid may be examined without preparation, a drop being squeezed out from the gland upon the slide, covered with a piece of thin glass, and viewed with a high power. It will be found to consist of a clear colourless liquid, with a few corpuscles, and, occasionally, portions of detached epithelium. THE LIVER. THE liver is composed of numerous small lobules, polyhedral in figure, varying in size, from half a line to one line in diameter, packed closely together, and connected by fine areolar tissue, intermixed with blood vessels, nerves, and lymphatics. The entire gland is invested by two distinct coverings, an outer or serous coat, derived from the peritoneum, and under this a thin capsule of connective tissue. In order to study the structure of the liver, a small portion of the healthy gland should be obtained from the butcher. The liver of the pig is best, as the lobules in it are most distinct. An ordinary lens will render the lobules quite evident upon the surface of the specimen without any preparation. They will also be observed in sections, on the cut surface. Each lobule shows the intra-lobular vein in its centre, as indicated in the drawing 1. Interlobular spaces, containing the larger interlobular branches of the The cells of the hepatic tissue are easily examined in a fresh specimen, by scraping the cut surface with the edge of a knife. Place the matter so collected in a drop of water on the slide, and apply the covering glass in the usual manner. Under a moderately high power the liver-cells will be seen as depicted below. Fig. 110. Hepatic cells of man; magnified 400 times. a. Normal cells. b. With c. With fat. The cells resemble ordinary squamous epithelium; they vary in size from Toso to 340 of an inch. In the fœtus two or more large nuclei and nucleoli will be observed, and, in many instances, cells multiplying by the endogenous process; but in the adult the nucleus is often absent. The liver-cells contain, besides the nucleus, granular matter and minute granules of fat. The large fat-globules often seen are the result of disease. The hepatic cells are enclosed in a delicate membrane, and arranged in radiating lines from the centre to the circumference of the lobule. Sections of the liver may be made by means of Valentin's knife, but they are not very instructive unless the gland has been previously injected. Such preparations are indispensable for the study of the vessels. The next drawing represents some of the tubes filled with hepatic cells. The liver of the pig is best for making this preparation. A small portion should be carefully teazed out, covered in the usual way, and examined with the high power. Fig. 111. a Termination of a small interlobular duct in the pig's liver, and communi- Bile does not generally contain any microscopic elements of importance. Epithelial cells from the gall-bladder will usually be observed, with small masses of colouring matter, and, occasionally, plates of cholesterine. THE THYROID GLAND. MICROSCOPIC examination of the thyroid body demonstrates the existence of numerous vesicles or loculi lined with epithelium, and distributed through a fibrous stroma. Preparation.-Select specimens of young subjects in preference-those from birds, dogs, and rabbits answer very well. Sections may be made by the double knife, or small portions may be teazed out in a little water on the slide, covered with the thin glass, and viewed with the high power. Several preparations may be necessary before the characteristic appearance indicated in the drawing is seen. Some gland-vesicles from the thyroid gland of a child; magnified 250 Injected specimens will be required, in order to study the disposition of the vessels, which are best seen in sections near the surface. In many specimens no defined epithelium will be distinguished; but the loculi or vesicles will be seen to be filled with a fluid containing numerous dark granules, probably the result of a post-mortem change. THE THYMUS GLAND. THE thymus gland is formed of a number of lobules united by connective tissue, and attached to a common stem in the centre of which a canal exists. The arrangement of the parts is shown in the accompanying sketch of the thymus of the calf. In man the central canal is of an irregular form. The duct generally runs in an irregular spiral course through the middle of the gland; upon its centre there are to be observed a number of openings leading into the several lobules. |