670 TEXTBOOK OP ZOOLOGY 



body fat, and a condition of hyperirritability of the nervous system. 

 It seems, therefore, that the difference between an overly energetic and 

 a sluggish person, and a lean and an obese person, may often be traced 

 directly to the degree of functioning of the thyroid gland. Hyper- 

 thyroidism is accompanied by increased excretion of calcium. It dif- 

 fers from the calcium upset due to parathyroid disturbance in that in 

 hyperthyroidism its concentration in the blood remains normal. 



Work on the lower vertebrate groups suggests the probability of 

 an important function of the thyroid gland in determining the hiber- 

 nation periods of certain animals. The thyroid performs additional 

 functions in conjunction with other of the endocrine glands; e.g., 

 the control of sexual activity; but these interrelationships will not 

 be discussed here. Peculiarly, thyroxin, although an accelerator of 

 oxidation in vertebrates, has a depressor effect on cell division and 

 differentiation in such invertebrates as Paramecium, sea urchin, and 

 the hydroid Pennaria. This has not been completely explained. 



The Parathyroid Glands 



Connected with the thyroid body are four little glands about the 

 size of small peas, so insignificant in appearance that they were 

 overlooked for many years. The removal of these small bodies along 

 with the thyroid gland in certain operations provoked such startling 

 results, however, as to attract attention to their presence and to 

 evoke considerable interest in their investigation. A complete re- 

 moval of the parathyroids results in unbalancing the blood calcium 

 and in a type of convulsion known as tetany; death is the usual result. 

 A deficient supply of paratJioriJione or parathyrin, the hormone of 

 the parathyroid glands, may be responsible for defective growth of the 

 bones and for deficient formation of enamel and dentine of the teeth. 

 Calcium is needed for both the teeth and the bones and the introduc- 

 tion of either calcium or parathormone into the body is made to sup- 

 plement a deficient supply of calcium due to malfunctioning of the 

 parathyroids. Removal of the parathyroids also brings about a fall 

 in the renal excretion of phosphorus, and the in.jection of parathor- 

 mone causes an immediate rise in the level of renal phosphorus. 

 Parathyroid activity is particularly useful to laying hens where so 

 much calcium is needed in shell formation. There is evidence of a 

 close functional relationship between the pituitary gland and the 

 parathyroid. In dogs the removal of the pituitary causes atrophy of 

 the parathyroids, particularly if the pancreas has been removed also. 



