218 



Basic Structure of Vertebrates 



HEP. PORTAL 



BILE DUCT 



HEPATIC A 



R. SUPRARENAL VES. 



R. RENAL VES 



VENA CAVA 



LEFT SUPRARENAL GLAND 



SUP MESENTERIC A, 



SPERMATIC COVARIAN) VES 



l "\V~;~\' - URETER 

 MESENTERIC A. 



Fig. 185. The adrenals or suprarenals shown in relation to the kidneys as seen 

 when the fat which normally encloses them is removed. The relations of the blood- 

 vessels which supply them are shown. (A.) Artery; (L.) left; (R.) right; (V.) vein. 

 (Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



The endocrine secreted by the medullary tissue of the adrenal is 

 epinephrine or adrenalin, its empiric chemical formula being 

 C9H13O3N. Like the sympathetic nerves, epinephrine has a stimulating 

 effect upon smooth muscle. It is so potent that its physiologic effects 

 appear even when it is diluted to 1 part in 400,000,000 of blood. 



The well-known experiments of W. B. Cannon led him to his emer- 

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

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

 animal is excited by pain, fear, or anger, epinephrine 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 prepared 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 epi- 

 nephrine is given off at the junction of a sympathetic nerve with its 

 effector. This substance has been called "sympathin." 



The adrenal gland has characteristic visceral innervation, being 

 entered by nerves from both the sympathetic and the parasympathetic 

 autonomic divisions. The medulla serves as an amplifier for the tho- 

 racolumbar division. A radio station may receive a message over one 

 telephone line and then broadcast the message to the world. The 

 medulla receives an impulse from a sympathetic nerve and thereupon 

 broadcasts epinephrine to the whole body. The embryonic origin of 



