THE THYROID GLAND OR BODY 1439 



masses of irregular form and size, known as lobes and lobules. Mere slender septa separate 

 the secretory alveoli from one another. While the anterior portion of the capsule is thin and 

 easily torn, the posterior portion is thick and dense. When the organ is cut into, it is of a 

 brownish-red color, and is seen to be made up of a number of closed vesicles or alveoli con- 

 taining a yellow glairy fluid and separated from each other by intermediate connective tissue. 



It is a compound tubular gland, each lobule of which consists of a number of short closed 

 tubules or alveoli, which are surrounded by the interstitial reticulum. 



According to Baber, who has published some important observations on the minute structure 

 of the thyroid, the vesicles of the thyroid of the adult animal are generally closed cavities; but in 

 some young animals (e. g., young dogs) the vesicles are more or less tubular and branched. This 

 appearance he supposes to be due to the mode of growth of the gland, and merely indicating that 

 an increase in the number of vesicles is taking place. Each vesicle is lined by a single layer of 

 cuboidal epithelial cells which rest upon a delicate basement membrane. Between the tubules 

 exists a delicate reticulum. The vesicles are of various sizes and shapes, and contain as a normal 

 product a viscid, homogeneous, semifluid, slightly yellowish material which frequently contains 

 red corpuscles in various stages of disintegration and decolorization, the yellow tinge being 

 probably due to the hemoglobin, which is thus set free from the colored corpuscles. This normal 

 product is known as colloid material, and it is secreted by the epithelium. What part if any the 



Vesicle. 



Lymphatic ressel..- 

 \ 41 



ir. 7 ? of glemd-veride. 



FIG. 1195. Minute structure of the thyroid. From a transverse section of the thyroid of a dog. Semi- 

 diagrammatic. (Baber.) 



colloid plays in the formation of the internal secretion of the gland is not known. It is quite 

 possible that the colloid corresponds to the external secretion of glands with ducts and that the 

 true internal secretion passes directly into the capillaries which form a network about the alveoli 

 (Szymonowicz), or passes into the lymphatics. In the thyroid gland of the dog, Baber has 

 found large round cells, parenchymatous cells, each provided with a single oval-shaped nucleus, 

 which migrate into the interior of the gland vesicles. Between the thyroid vesicles in the human 

 being arecollections of round cells. They are, in reality, miniature immature vesicles, and are 

 much more numerous in youth than in old age. 



The capillary bloodvessels form a dense plexus in the connective tissue around the vesicles, 

 between the epithelium of the vesicles and the endothelium of the lymph spaces, which latter 

 surround a greater or smaller part of the circumference of the vesicles. These lymph spaces 

 empty themselves into lymphatic vessels which run in the interlobular connective tissue, not 

 uncommonly surrounding the arteries which they accompany, and communicate with a network 

 in the capsule of the gland. Small glands may be connected with this network. Baber has 

 found in the lymphatics of the thyroid a viscid material which is morphologically identical with 

 the normal constituent of the vesicle. 



Vessels and Nerves. The arteries (Figs. 447 and 499; see also p. 592) supplying the thy- 

 roid are the superior thyroid from the external carotid, and the inferior thyroid from the thyroid ) / 

 axis of the first part of the subclavian. Sometimes there is an additional vessel, the thyroidea ' 

 media or ima, usually arising from the innominate artery, but sometimes from the arch of the 

 aorta or the common carotid. It ascends upon the front of the trachea. The superior thyroid 



