THE ENDOCRINAL ORGANS 



459 



bility and "touchiness." The injection of parathyroid extract into the 

 blood is followed by a rapid increase in blood calcium, so rapid that a slight 

 overdose may cause death, for the chemical balance of the blood must be 

 delicately regulated to maintain normal health. 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 

 hardness and become fibrous. 



Fig. 381. — Diagram to show the derivatives of the branchial pouches. le, He, Hie. 

 IVe, Ve, external branchial grooves; li. Hi, Hit, IVi, internal branchial pouches; 

 TA, auditory tube and tympanic cavity; Tons., palatine tonsil; EpIII, EpIV, para- 

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

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



Since calcium and phosphorus metabolism are also dependent upon 

 the presence of vitamin D in the food, the problem of calcium balance is 

 not simply a problem of endocrinology. 



In brief, the action of the parathyroid endocrine resembles that of a 

 catalyst which facilitates the manufacture of calcium salts in the body 

 and thus helps to regulate the irritability of the nerves. 



Development. In the human embryo, the parathyroid glands are 

 formed from the epithelium of the third and fourth pharyngeal pouches. 

 By proliferation from the dorso-lateral wall of these pouches are formed 

 masses of cells which soon lose connexion with the pouches and migrate 

 caudally. In this migration they become closely associated with the 

 anlagen of the thymus gland. The cells of the third pouch are carried 



