SECRETIONS 



99 



Fig. 55. — The thyroid 



thyroids. In addition, there are more or less isolated cells, such as 

 those that produce secretin. 



The thyroid gland is located on the sides of the trachea pos- 

 terior to the larynx (Fig. 55). Its removal causes death in all cases 

 after a few weeks. However, death can be prevented, and normal 

 function secured after removal, by grafting in a thyroid from 

 another animal. This grafting can be 

 done successfully in any region of the 

 body that affords an abundant blood 

 supply. Injection of thyroxin, as the 

 endocrine of this gland is called, or feeding 

 of thyroid will also prevent death after 

 thyroidectomy, but must be repeated at 

 regular intervals in order to maintain life. 

 Extreme deficiency of thyroid secretion 

 in children results in the disease known 

 as cretinism (Fig. 56) in which neither 

 physical nor mental development is normal. 

 Cretinism can be remedied bv admin- 



istration of the endocrine from other gland of man showing iti- 

 animals if this treatment is started before position with reference to 

 the individual is too old. Subnormal ^^^ ^'^^-^'^^ ^^'^ trachea, 

 thyroid secretion in adults causes a con- ■^■larynx; r, trachea; Thy, 

 dition known as myxoedema, strikingly 



characterized by swollen, distended, dry skin, and resulting 

 in mental impairment. Here again, administration of extracts 

 is helpful in some cases. On the other hand, the thyroid 

 may over-secrete; in extreme cases this results in exophthalmic 

 goiter, characterized by enlarged thyroid, rapid pulse, moist skin, 

 protruding eye-balls, and nervous symptoms of excitability and 

 restlessness. This condition may be remedied by removal of a 

 part of the gland so that the amount of secretion is reduced to 

 normal. 



The adrenal bodies, which in the frog lie on the ventral surface 

 of the kidneys, and in higher vertebrates come to lie anterior to 

 the kidneys, are composed of two regions, a central medulla and 

 a surrounding cortex (Fig. 57). These two regions differ in func- 

 tion, but both are seats of endocrine secretion. The secretion 

 of the medulla, known as adrenalin, is important in increasing 

 the flow of blood carrying augmented sugar supply in cases of 



