410 



Till-: ORGANS 



The Parathyreoids 



These are small ductless glands which usually lie upon the posterior 

 surface of the lateral lobes of the thyreoid. There are commonly 

 two pairs, a superior and an inferior, on each side. The number is, 

 however, subject to variation. Each gland is from 6 to 8 mm. long, 

 about half that in ])readth, and 2 mm. thick. Small groups of cells 

 having the structure of the parathyreoids have been found below the 

 thyreoid and within the thyreoid and thymus. 



Fig. 289. — Section of Human Parathyreoid; showing mainly "clear," "principal,' 



or "chief" cells. (Pool.) 



The parathyreoid is surrounded by a thin connective- tissue capsule 

 which sends a variable amount of connective tissue into the gland as 

 septa. When the amount is considerable the gland shows a sub- 

 division into lobules. The stroma consists largely of reticular tis- 

 sue and is very vascular. The number and arrangement of the cells 

 vary. The gland may be almost wholly cellular with very little con- 

 nective tissue, the groups of cells may be widely separated by inter- 

 stitial tissue, or there may be any intermediate condition. The cells 

 are arranged in irregular groups or cords (Figs. 289, 290) sometimes 

 around tubules, sometimes having a distinctly alveolar structure (Fig. 



