198 ESSENTIALS OF ZOOLOGY 



America; but the present understanding of the hormone and its use 

 has made possible the prevention and cure of most cases of this nature. 



Improper functioning of the gland in adult years results often 

 in a condition known as myxedema, in which there is a thickening 

 and drying of the skin, a puffiness of the eyelids and lips, loss of 

 hair due to the condition of the skin, a slowing down of metabolism 

 and heartbeat, a depression of body temperature, the deposition of 

 large quantities of fat, and a final result, in many cases, of im- 

 becility. The administration of thyroxine, especially in the early 

 stages, accomplishes a complete, or at least a temporary, remedy. 



An overfunctional thyroid gland, in which the condition is known 

 as hype7'thyroidism, results in an increased metabolic rate, a loss of 

 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 hj^droid 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 parathormone or parathyrin, the hormone of the 

 parathyroid glands, may be responsible for defective growth of the 



