nies the pulmonary artery and passes directly into the bronchial glands at the hilum of the lung."

Accompanying the bronchi and bronchial arteries are found numerous nerve-fibers, of the nonmedullated and medullated varieties, arranged in bundles of varying size, in the course of which are

Fig. 239.-From section of human lung stained in orcein, showing the elastic fibers surrounding the alveoli.

found sympathetic ganglia. Berkley (94), who has studied the distribution of the nerves of the lung with the chrome-silver method, finds that in the external fibrous layer of the bronchi is found a plexus of very fine and of coarser fibers, from which branches are

given off which end in the muscle tissue of the bronchi, and others which pass through this layer to form, after further division, a subepithelial plexus from which fibrils may be traced into the connective-tissue folds in the larger bronchi and between the bases of the epithelial cells in the smaller bronchi and bronchioles. Some few fibrils were traced between alveoli situated near bronchi, "terminating, apparently, immediately beneath the pavement epithelium in an elongated or rounded minute bulb;" these may, however, represent endings on nonstriated muscle tissue. The bronchial arteries have an exceedingly rich nerve supply.

E. THE THYROID GLAND.

The thyroid gland is developed from three sources: Its middle portion, the isthmus of the gland, originates as a diverticulum of

Lumen of follicle.
Connective tissue.

Epithelium of follicle.

Fig. 241. From section through thyroid gland of child.

the pharyngeal epithelium, from what is later the foramen cæcum of the tongue; both lateral portions, the right and left lobes, are formed from a complicated metamorphosis of the epithelium of the fourth visceral pouch. These various parts unite in man into one, so that in the adult the structure of the organ is continuous. The thyroid gland consists of numerous noncommunicating acini or follicles of various sizes lined by a nearly cubic epithelium; the lobules are separated from each other by a highly vascularized connective tissue, continuous with the firm connective-tissue sheath surrounding the whole gland. The follicles are either round, polyhedral, or tubular, and are occasionally branched (Streiff). At an early stage the acini are found to contain a substance known as 'colloid" material (vid. Technic).

64

Langendorff has shown (vid. Technic) that two varieties of cells exist in the acini of the thyroid body-the chief cells and colloid

cells. Those of the first variety apparently change into colloid cells, while the latter secrete the colloid substance. During the formation of this material the colloid cells become lower, and their entire contents, including the nuclei, change into the colloid mass. Hürthle distinguished two processes of colloid secretion; in the one the cells remain intact, in the other they are destroyed. He claims that the colloid cells of Langendorff participate in the former process, while in the latter they are first modified (flattened) and then changed into the colloid substance. The colloid material may enter the lymph-channels, either directly by a rupture of the acini, or indirectly by a percolation of the substance into the intercellular clefts, whence it is carried into the larger lymphatics.

Anderson (91) and Berkley (94) have studied the distribution of the nerve-fibers of the thyroid gland with the chrome-silver method; the account given by the latter is the more complete and will be followed here. The nonmedullated nerves entering the gland. form plexuses about the larger arteries, which are less dense around the smaller arterial branches. Some of these nerve-fibers are vascular nerves and end on the vessels; others form perifollicular meshes surrounding the follicles of the gland. From the network of nervefibers about the follicles, Berkley was able to trace fine nerve filaments which seemed to terminate in end-knobs on or between the epithelial cells lining the follicles. Even in the best stained preparations, however, not nearly all the follicular cells possess such a nerve termination. In methylene-blue preparations of the thyroid gland (Dr. De Witt) some few medullated fibers were found in the nerve plexus surrounding the vessels. In a number of preparations these were traced to telodendria situated in the adventitia of the vessels, showing that at least a portion of these medullated nerves are sensory nerves ending in the walls of the vessels.

PARATHYROID GLANDS.

Small glandular structures found on the posterior surfaces of the lateral lobes of the thyroid were discovered by Sandström in 1880. They are surrounded by a thin connective-tissue capsule and divided into small imperfectly developed lobules by a few thin fibrous-tissue septa or trabeculæ, which support the larger vessels. The epithelial portions of these structures consist of relatively large cells and capillary spaces. According to Schaper (95), who has recently subjected these structures to a careful investigation, the epithelial cells have a diameter which varies from 10 to 12, possessing nuclei 4 μ in diameter. These cells are of polygonal shape and have a thin cell-membrane, a slightly granular protoplasm, and a nucleus presenting a delicate chromatic network. The cells are arranged either in larger or smaller clusters or, in some instances, in anastomosing trabeculæ or columns, consisting either of a single row or of several rows of cells. Between the clusters or columns of cells are found rela

tively large capillaries, the endothelial lining of which rests directly on the epithelial cells. Connective-tissue fibrils do not, as a rule, follow the capillaries between the cell-masses. The structure of the parathyroid resembles in many respects that of certain embryonic stages of the thyroid, and it has been suggested that these bodies represent small masses of thyroid gland tissue, retaining their embryonic structure. Schaper has observed parathyroid tissue, the cells of which were here and there arranged in the form of small follicles, some of which contained colloid substance. Such observations lend credence to the view regarding the parathyroid as an embryonic structure. Whether in this stage they form a special secretion has not been fully determined. (See Schaper, 95.)

263. For the demonstration of the larynx and trachea, young and healthy subjects should be selected. Pieces of the mucous membrane or the whole organ should be immersed in a fresh condition. Sections through the entire organ present only a general structural view; but if a close examination of accurately fixed mucous membrane be desired, the latter should be removed with a razor before sectioning and treated separately.

264. Chromic-osmic acid mixtures are recommended as fixing agents, and safranin as a stain. Besides the nuclear differentiation, the goblet cells stain brown, and the elastic network of the stratum proprium and the submucosa a reddish-brown.

For examining the epithelium, isolation methods are employed, such as the alcohol of Ranvier (T. 128).

265. The examination of the respiratory epithelium is attended with peculiar difficulty; it is, perhaps, best accomplished by injecting a 0.5% solution of silver nitrate into the bronchus until the lumen is completely filled, and then placing the whole in a 0.5% solution of the same salt. After a few hours, wash with distilled water and transfer to

70% alcohol. Thick sections are now cut and portions of the respiratory passages examined; the silver lines represent the margins of the epithelial cells. Such sections should not be fastened to the slide with albumen, as the latter soon darkens and blurs the picture. These specimens may also be stained.

266. For the elastic fibers, especially those of the alveoli, fixation in Müller's fluid (T. 27) or in alcohol and staining with orcein is a good method. Fresh pieces of lung tissue treated with potassium hydrate show numerous isolated elastic fibers.

267. Pulmonary tissue may be treated by Golgi's method, which brings out a reticular connective-tissue structure in the vessels and alveoli (vid. T. 252, Oppel).

268. The pulmonary vessels may be injected with comparative ease. 269. The thyroid gland is best fixed in Flemming's solution; it is then stained with M. Heidenhain's hematoxylin solution or, better still, with the Ehrlich-Biondi mixture which differentiates the chief from the colloid cells; the former do not stain at all, while the latter appear red with a green nucleus (Langendorff). The colloid substance of the thyroid gland does not cloud in alcohol or chromic acid, nor does it coagulate in acetic acid, but swells in the latter; 33% potassium hydrate hardly causes the colloid material to swell at all, though in weaker solutions it dissolves after a long time.

V. THE GENITO-URINARY ORGANS.

A. THE URINARY ORGANS.

1. THE KIDNEY.

THE kidney is a branched tubular lobular gland, which in man consists of from ten to fifteen nearly equal divisions of pyramidal shape known as the renal lobes. The apex of each pyramid (the Malpighian pyramid) projects into the pelvis of the kidney.

kidney is surrounded

by a thin but firm capsule consisting of fibrous connective tissue containing a few elastic fibers and, in its deeper portion, a thin layer of nonstriated muscle-cells.

Vein.

a

The

Fig. 243. Kidney of new-born infant, showing a distinct separation into reniculi; natural size. At a is seen the consolidation of two adjacent reniculi.

The secreting portion is composed of a large number of tubules twisted and bent in a definite and typical manner, the uriniferous tubules. In each one of these tubules we distinguish the following segments: (1) Bowman's capsule, or the ampulla, surrounding a spheric plexus of capillaries, the glomerulus, which, with the capsule of Bowman, forms a Malpighian corpuscle;