518 VERTEBRATE LIFE AND ORGANIZATION 



The adrenal medulla, in contrast to most other endocrine glands, 

 is not essential for life; its removal does not cause a deficiency disease. 

 This gland is believed to secrete a small amount of epinephrine and 

 norepinephrine continuously; the rate of secretion is under nervous 

 control. 



It is widely believed that the secretion of the adrenal medulla 

 functions during emergencies to reinforce and prolong the action of 

 the sympathetic nervous system. There is good evidence that epineph- 

 rine secretion is greatly increased by stresses such as cold, pain, trauma, 

 emotional states, and certain drugs. The changes resulting from the 

 action of the sympathetic nerves and epinephrine would prepare an 

 animal to attack its prey, defend itself against enemies, or run away. 

 These include the following: (1) the efficiency of the circulatory system 

 is increased by increased blood pressure, heart rate, and the dilation 

 of the large blood vessels; (2) the increase in the ability of blood to coag- 

 ulate and the constriction of the vessels in the skin tend to minimize 

 the loss of blood if the animal is wounded; (3) the intake of oxygen is 

 increased by increased rate of breathing and dilation of the respiratory 

 passages; (4) the mobilization of the glycogen stores of the liver and 

 muscle makes glucose available for energy; and (5) the release of ACTH 

 from the pituitary is stimulated (p. 625). The ACTH in turn stimu- 

 lates the release of glucocorticoids from the adrenal cortex which in- 

 crease the breakdown of protein and make further carbohydrate 

 available. 



Epinephrine is widely used clinically in treating asthma (it dilates 

 respiratory passages), in increasing blood pressure, and in stimulating a 

 heart that has stopped beating. 



The adrenal cortex is more complex than the medulla both histologi- 

 cally, for it is composed of three layers of cells, and functionally, for it se- 

 cretes a number of hormones with different types of activity. The cortex 

 is composed of three zones: an outer glomerulosa, a middle fasciculata 

 and an inner reticularis (Fig. 30.5). Cells are formed by mitosis in the 

 outer layer and are pushed inward to the reticularis, where they degen- 

 erate and disappear. The cells of the fasciculata are believed to be most 

 active in hormone production. The embryos of man and other mammals 

 have very large adrenals— as large as the kidneys— which result from the 

 presence of a large mass of cells, the fetal zone, interposed between the 

 cortex and medulla. The fetal zone regresses and disappears after birth. 



Some thirty different hormones have been extracted from the adrenal 

 cortex of various species; all belong to the class of chemicals called 

 steroids, to which the male and female sex hormones also belong. No 

 single one of these hormones is the physiologic equivalent of whole 

 adrenal extract, and a mixture of at least two of them must be injected 

 if the glands have been removed. The cortical hormones have been 

 grouped into three categories, although there is some overlapping. These 

 are: (1) glucocorticoids, which stimulate the conversion of proteins to 

 carbohydrates, (2) mineralocorticoids, which regulate sodium and po- 

 tassium metabolism, and (3) androgens, which have male sex hormone 

 activity. The most potent glucocorticoid is hydrocortisone (Compound 



