112 HUMAN 



PHYSIOLOGY 



the usual amount may cause a suspension of breath- 

 ing. This favors the view that carbon dioxid is the 

 normal stimulant for the respiratory center. It will 

 be convenient to postpone a fuller treatment of this 

 subject until the physiology of respiration is taken up. 



Other services of the medulla are connected with the 

 regulation of the circulation. It will be recalled that 

 the vagus nerves spring from the medulla and these 

 nerves contain many fibers which extend to and from 

 the heart. Destruction of the medulla will not stop 

 the beating of the heart as it stops the breathing but 

 the heart will be deprive^rbf a certain control by the 

 operation. The influence withdrawn is mainly of an 

 inhibitory sort and the result is a quickening of the 

 heart rate. 



Besides exercising a regulating function upon the 

 heart the medulla is responsible for the maintenance 

 of the tone of the blood-vessels. This fact is expressed 

 by saying that the medulla contains the vasomotor 

 center. To destroy the region to which this name is 

 applied is to cause a nearly universal slackening of the 

 small arteries and veins, showing that they were pre- 

 viously held in a state of sustained contraction. Vaso- 

 motor reflexes can best be studied in connection with 

 the other problems of the circulation. 



The medulla is the seat of still other mechanisms, 

 some of which pertain to the alimentary system. It 

 has to do with the execution of swallowing, an act 

 which is more complex than we are apt to appreciate. 

 It is concerned in the government of the salivary glands 

 and the glands in the lining of the stomach. And we 

 must not forget that the medulla is also a part of the 

 main line of communication between the higher parts 

 of the brain and the body. It is traversed by the fibers 

 which we employ in voluntary acts and by those which 

 make possible the sensations which we refer to the 

 trunk and extremities. Many of the paths of trans- 

 mission which lie in the medulla cross from one side of 



