THE PAEATHYROID GLANDS 563 



a size as those of the thyroid itself. The cell columns without colloid 

 are more numerous than in the thyroid gland, giving to the aberrant 

 bodies a decidedly cellular appearance. Each aberrant mass is usually 

 inclosed by a very thin connective tissue capsule which sends delicate 

 processes between the cell groups. The epithelial cells retain all the 

 characteristics of those of the thyroid gland, and can be readily distin- 

 guished from the epithelium of the parathyroid glands with which the 

 accessory thyroid bodies have been frequently confused. They are also 

 much less vascular than the parathyroids. 



III. THE PARATHYROID GLANDS 



The PARATHYROIDS, or epithelial bodies, are small glandular bodies 

 of irregular distribution, usually found in relation with the posterior 

 margin of the lateral lobes of the thyroid gland. They arise as buds from 

 the dorsal pockets of the third and fourth pharyngeal pouches. Typi- 

 cally four parathyroids are present, the superior pair situated near the 

 lateral, the inferior near the median margin of the lateral lobes of the 

 thyroid. The inferior pair in the adult develops from the anterior pair 

 of anlages, and frequently becomes embedded in the thyroid tissue. They 

 may occur also in relation with the tracheal or laryngeal wall and may 

 be found as high as the hyoid bone or as low as the border of the thymus. 

 They also vary greatly in size and shape, but usually are of ovoid form 

 and about three to five millimeters in diameter. 



Each parathyroid is invested by a thin capsule of dense connective 

 tissue and consists of a mass of epithelial cells supported by a delicate 

 fibrous reticulum. The epithelial cells are of two chief types, desig- 

 nated by Welsh (Jour. Anat. and Physiol., 1898) as the 'principal* and 

 the 'oxyphil' or acidophil cells-. 



The principal cells are the more abundant. They are ovoid or 

 spheroidal elements, with a clear vesicular cytoplasm, a distinct cell 

 membrane, and a large spherical nucleus, whose chromatin is irregularly 

 distributed and often gives the nucleus a somewhat vesicular character. 



The acidophil cells are of similar shape but are provided with a 

 small spherical nucleus, which is very rich in chromatin, and a granular 

 acidophilic cytoplasm. The acidophil are less numerous than the 

 principal cells. 



The distribution of the epithelial cells is subject to considerable vari- 

 ation. Most frequently they form an almost solid epithelial mass, in 



