234 The Vertebrate Organ Systems 



THE ADRENAL MEDULLA 



Embryologically the medulla has the same origin as the sympa- 

 thetic ganglia of the autonomic nervous system. As might be inferred 

 from this common origin, the action of the medulla secretion, adren- 

 aline, is very similar to that of the sympathetic ganglia. 



The Function of the Adrenal Medulla. — When adrenaline is 

 released, it has many marked effects on various bodily functions. 

 Among these effects are the following : the blood sugar level is raised, 

 the muscles of the intestinal tract slow down their movements, the 

 blood supply to the striated muscles is increased, the pupil of the eye 

 dilates, the heart rate is increased, the coagulability of the blood is 

 increased, and the spleen contracts to release additional erythrocytes 

 into the blood stream thus increasing the oxygen-carrying capacity of 

 the blood. From this listing it might appear that adrenaline is nec- 

 essary for the normal functioning of the animal; however, following 

 extirpation of the medulla, the animal is able to live an apparently 

 normal life. 



For this reason, there has been much speculation as to the exact 

 role played by this gland. One theory is that the medulla plays an 

 important part in equipping the animal to meet emergency situations. 

 The changes as listed above clearly would help an animal to be more 

 alert and to be better able to defend itself or to escape. This emergency 

 theory was once widely accepted as the most reasonable explanation 

 of the function of this gland. Now, however, it is realized that the 

 sympathetic system is able to perform all these functions alone ; per- 

 haps the adrenal medulla only exerts a tonic or conditioning influence 

 upon the organs supplied by the sympathetic system. 



Adrenalin is used extensively in medical practice because of its 

 specific effects on blood pressure, blood coagulability, heart rate, etc. 

 In disorders such as asthma where the bronchial musculature is con- 

 tracted. Adrenalin causes relaxation and thus gives relief. Its stimu- 

 latory effect on heart muscle is often utilized extensively in cases of 

 drowning, shock, and other accidents. 



The Composition of Adrenaline. — Adrenaline or epinephrine is 

 another of the better known hormones. It has a relatively simple 

 formula, C9H13NO3, which has been known since 1905. This hormone 

 is extremely potent and even at great dilutions has pronounced effects. 



Little is known of the possible effects of hypo- or hypersecretion 

 of this gland. 



