328 



CHORDATE ANATOMY 



cortin, of unknown chemical composition, which is essential to life. 

 Destruction of the cortex is followed by Addison's disease, which is char- 

 acterized by a deep pigmentation of the skin and great weakness. Death 

 comes rapidly unless cortin is administered. The cortex of the adrenal is, 

 relatively to the medulla, largest during foetal life, and there is Kttle doubt 

 that its secretions have a strong influence upon growth. 



The endocrine secreted by the medullary tissue of the adrenal is 

 epinephrine or adrenin, its empirical chemical formula being C9H13O3N. 

 Like the sympathetic nerves, adrenin has a stimulating effect upon smooth 

 muscle. It is so potent a drug that its physiological effects appear even 

 when it is diluted to one part in 400,000,000 of blood. 



<XOM£»ULOS». 



■ZONA FASCICULATA, 



ZONA RETICULARIS. 



Fig. 292. — A portion of a section of the adrenal gland as seen under low-power 

 microscope, showing the differentiation into cortical and medullary regions. Different 

 endocrines are secreted by these two tissues, which have a different embryonic origin. 



Cannon's well-known experiments upon animals led him to his emer- 

 gency theory of adrenal action. He found that, in a quiet undisturbed 

 animal, adrenin is absent from the blood. When, however, the animal 

 is excited by pain, fear, or anger, adrenin increases. As a result, the 

 heart beats more strongly, breathing becomes deeper and more rapid, 

 intestinal action ceases, the liver releases sugar more rapidly, the muscles 

 respond more quickly to stimulation, the tonus of the blood-vessels is 

 raised, the coagulability of the blood increases, and so the animal is pre- 

 pared either to run or fight. Thus the medullary secretion reinforces the 

 action of the sympathetic nerves. 



Cannon has shown also that a substance which resembles adrenin in 

 its effects is given off by organs stimulated by the sympathetic nervous 



