THE THYROID GLAND. 



3 2I 



PARATHYROID GLANDS. 



Small glandular structures found on the posterior surfaces of the 

 lateral lobes of the thyroid were discovered by Sandstrom 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 trabeculae, which support the larger vessels. The epithelial 

 portions of these structures consist of relatively large cells and capil- 

 lary 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 /i to 12 //, possessing nuclei 4 // 

 in diameter. These cells are of polygonal shape and have a thin 

 cell-membrane, a slightly granular protoplasm, and a nucleus pre- 

 senting a delicate chromatic network. The cells are arranged either 

 in larger or smaller clusters or, in some instances, in anastomosing 

 trabeculae or columns, consisting either of a single row or of several 

 rows of cells. Between the clusters or columns of cells are found rela- 



''W&glp* 



Fig. 258. From parathyroid of man. 



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. These vessels may 

 therefore be regarded as sinusoids (Minot). 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 em- 

 bryonic 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 obser- 

 vations 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.) 



