332 



CHORDATE ANATOMY 



lie dorsal to the thyroid, but may be occasionally enclosed within its 

 connective- tissue capsule. This relation, however, is purely topographical; 

 there is no functional similarity. Their blood supply is from the inferior 

 thyroid arteries. 



Unlike the thyroids, the parathyroids are formed of masses and cords 

 of polygonal epithelial cells, among which numerous blood-vessels are 

 interspersed. Colloid-filled follicles are rare; but they multiply in number 

 when the thyroid is removed, and may take over the function of the thyroid 

 in an emergency. While, therefore, an animal may lose its thyroid glands 

 without necessarily fatal consequences, the removal of the parathyroids 



Ub. lY?j:. 

 Thymus 

 Fig. 295. — Diagram to show the derivatives of the pharyngeal pouches. le, He, 

 I He, IVe, Ve, external pharyngeal grooves; It, Hi, Hit, IVi, internal pharyngeal 

 pouches; TA, auditory tube and tympanic cavity; Tons., palatine tonsil; EpHI, 

 EpIV, parathyroid glands; Ub, ultimobranchial body; Th., thyroid gland. D.th.gl., 

 ductus thyroglossus. (From Morris, after Keibel and Mall.) 



is followed by convulsions and death unless parathyroid extract is adminis- 

 tered intravenously. There is a marked fall in the calcium content of the 

 blood, followed by cramps and muscular tetany. Apparently calcium 

 salts act as a nerve sedative to prevent acute stimulation of muscular 

 contraction. The chemical nature of the parathyroid endocrine is not 

 yet known. 



On the other hand, even a slight over-activity of the parathyroids may 

 have serious consequences. The reserve supply of calcium for bodily use 

 is limited, and excess of parathyroid endocrine in the blood may cause 

 the withdrawal of calcium from the bones and teeth which lose their 

 harflness and l)ecome fibrous. 



