2.58 



THE ENDOCRINE GLANDS 



THE PARATHYROID GLANDS. 



In cyclostomes these ();laiKls occur as small masses of epithelioid 

 tissue on the ventral portion of each of the seven pairs of gill pouches. 

 In lizards, birds, and mammals they occur as two pairs of small 

 glandular structures. In development, small masses of epithelial 

 cells form as dorsal diverticula from the third and fourth pairs of 

 gill i)ouches. As the pharyngeal pouches are obliterated by further 

 growth, the four parathyroid anlages separate from their place of 

 origin, migrate l)ackward, and become embedded in the thyroid 

 lobes. (Fig. 159.) Histologically each gland is composed of cords 



Fig. 159. — Photograph of a section through the thyroid (right) and parathyroid 



(left) of the dog. 



of epithelioid cells, with a capillary network in the connective tissue 

 between them. Two types of cells have been described — chief cells, 

 which are large and pale, and still larger acidophil cells (chromo- 

 philic) with granular cytoplasm. The cells are arranged in small 

 masses or cohunns (Fig. KH)) or ])ossibly follicles, in which colloid 

 without iodine collects. In parathyroids of old animals, a(li])ose 

 tissue may develop in the connective tissue between the ei)itheli()id 

 cells. There is no regeneration of })arathyroids after their removal. 

 Removal of ])ortions of the parathyroids causes hyperirritability 

 of the nervous system and sense organs. There is an accompanying 

 decrease in the calcium content of th(> blood. The nornud seci-etion 

 is essential to normal calcium metabolism, uictabolisui of sugar. 



