228 



HISTOLOGY 



colloid material is not clear. The finding of two sorts of thyreoid cells, 

 one of which produces colloid, and the other does not, lacks confirmation. 

 The cells may exhibit refractive, secretory granules, which are larger and 

 coarser toward the free surface. Eosinophilic granules have been re- 

 ported, and in certain animals other granules of fatty nature have been 

 found, especially near the basal surface. Since the terminal bars are said 

 to be deficient at the angles where the epithelial cells meet, an opportunity 

 is afforded for the contents of the follicles to pass out between the epithelial 

 cells to the vascular tunica propria. 



The thyreoid follicles are surrounded by loose elastic connective tissue, 

 said to be reticular near the follicles, which contains very many blood and 

 lymphatic vessels in close relation with the epithelium. Denser connective 

 tissue forms a capsule and lobular partitions. It contains small arteries, 

 the media and intima of which are said normally to present local thicken- 

 ings (Fig. 216). The nerves from the cervical sympathetic ganglia form 

 perivascular plexuses, and pass to the follicles. 



PARATHYREOID GLANDS. 



It is generally stated that there are four parathyreoid glands in man, 

 the anterior or upper pair being derived from the fourth pharyngeal 



FIG. 217. SECTION OF A HUMAN PARATHYREOID GLAND. (Huber.) 



pouches, and the posterior or lower pair from the third (Fig. 208). They 

 are therefore entodermal structures. In the adult they are round or oval 

 bodies, said to measure from 3 to 13 mm., found on the dorsal or tracheal 

 surface of the thyreoid gland. They may be imbedded in its capsule or 

 attached to it by pedicles. Sometimes they (the lower pair?) are found 

 in the thymus. The parathyreoid glands may be lacking on one side, 

 where in other cases as many as four have been recorded ; they may atrophy 



