604 PHYSIOLOGY OF THE BRAIN-STEM 



and these impulses have a large part in the regulation of the important proc- 

 esses going on in these and other organs. 



It follows that the medulla exercises a determining influence on the fol- 

 lowing functions : secretion of saliva, movements of the tongue, swallowing, 

 movements of the stomach and intestine, vomiting, secretion of the gastric 

 and pancreatic juices; force and frequency of the heart beat, vascular tonus, 

 and regulation of the blood flow; respiratory movements and movements of 

 the larynx; also, to a certain extent, at least, the heat regulation of the body 

 both through the blood vessels and the skeletal muscles. In short, digestion, 

 circulation, respiration and heat regulation are all to a considerable extent 

 dependent upon the medulla. 



Some of these functions are, it is true, relatively simple ; for example, the 

 reflex secretion of saliva is on a plane with the simpler reflexes from the 

 spinal cord. But others, and, in fact, most of them, are very complicated, as 

 a critical study of the processes of swallowing, breathing, and distribution of 

 blood to the different organs will readily convince one. 



Moreover, experiment has shown that the different nerves act not only upon 

 the efferent nerves belonging to the same organ, or organ system, but that their 

 influence extends to other organ systems. Thus, e. g., by swallowing repeatedly 

 the heart beats are at first quickened, but subsequently are lowered to a rate below 

 the original; the vascular tonus decreases; the expiratory phases of respiration 

 last longer; labor pains become weak or cease altogether; hiccoughing may be 

 stopped by repeated swallowing, etc. (Kronecker and Meltzer). 



When we remember that in addition to exercising this multifarious con- 

 trol over the functions enumerated above, the medulla is the afferent pathway 

 of impulses from the cord to the higher parts of the brain, and of efferent 

 impulses from the latter to the former, it is apparent at once that it con- 

 stitutes an organ absolutely indispensable to life. The immediate cause of 

 death by destruction of the medulla is stoppage of the respiration, and by 

 keeping up this function artificially, life can be prolonged somewhat. But 

 this is not sufficient to maintain life indefinitely, for other disturbances, espe- 

 cially that of the heat regulation, are enough to bring on death within a 

 relatively short time. 



On the other hand, it can be shown that even a mammal can survive 

 section of the brain at the upper edge of the medulla, provided its continuity 

 with the spinal cord be not interrupted, without immediate danger to life and 

 without resort to artificial respiration or anything of the kind. Since, how- 

 ever, such an animal cannot move about or take nourishment, it is of course 

 impossible to keep it alive very long. 



Frogs, which because of the much lower intensity of their metabolism 

 require very little food and hence can go a long time without any food, live 

 much longer when deprived of the brain above the medulla. In fact, Shrader 

 succeeded in removing from frogs everything down to the medulla inclusive 

 of the cerebellum and the most anterior part of the medulla itself, and in 

 keeping them alive for four months. The most striking thing about the 

 behavior of these animals was an apparently irresistible impulse to be con- 

 stantly moving. 



