222 Basic Structure of Vertebrates 



vital processes, too little slows them down. Basal metabolism becomes 

 low, and the tendency is to "put on fat." In the young, both growth 

 and development are retarded, and, if the deficiency is very great, a 

 child, unless given thyroid artificially, may become an idiotic dwarf. 



Parathyroid Glands 



These glands, found in all vertebrates except fishes, develop by 

 outgrowth from the endodermal wall of the pharyngeal pouches of 

 the third and fourth pairs (Fig. 187). In the adult they are a pair of 

 small glands lying close to the thyroid or sometimes inside the con- 

 nective-tissue capsule enclosing the thyroid. Functionally, however, 

 they are quite unlike the thyroid. 



The parathyroids consist of masses and cords of the originally 

 epithelial cells, among which numerous blood-vessels are interspersed. 

 Histologically, they resemble lymph-nodes (Fig. 67). 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 emer- 

 gency. An animal may lose its thyroid gland without necessarily fatal 

 consequences, but removal of the parathyroids is followed by convul- 

 sions and death unless parathyroid extract is administered intra- 

 venously. There is a marked decrease in the calcium of the blood, 

 followed by cramps and muscular tetany. Apparently calcium salts 

 prevent acute stimulation of muscular contraction. 



Even a slight excess of 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 then lose 

 their hardness and become fibrous. 



The chemical nature of the parathyroid endocrine is not yet known. 



Thymus Gland 



In the cyclostome eel, Petromyzon, small thymus outgrowths 

 develop from the dorsal regions of all seven pairs of pharyngeal 

 pouches. In other vertebrates the number is more or less reduced, the 

 derivatives of the third and fourth pairs (Figs. 187, 189) usually per- 

 sisting or, in some mammals, the derivatives of only the fourth pair. 

 In fishes the thymus bodies of each side usually fuse to form an 

 anteroposteriorly elongated gland lying dorsal to the branchial cham- 

 bers. In amphibians and reptiles they are situated laterally in the 

 region of the neck. In mammals they lie at the base of the neck and 

 in the partition (mediastinum) between the right and left pleural 

 cavities — therefore close to the heart. These glands in mammals are 

 the "throat sweetbreads." 



