114 HUMAN PHYSIOLOGY 



the heart will be deprived of 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 The influence withdrawn is mainly of an 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 

 the mid-plane to the other while passing through. This 

 has to do with a prominent and curious truth that the 

 right side of the cerebrum sustains relations chiefly with 

 the left side of the body and vice versa. 



The limited portion of the brain-stem which inter- 

 venes between the medulla and the cerebrum is of 

 interest to us as containing way-stations on the paths 

 from the eyes and the ears to the higher centers. This 

 part of the brain is also known to preside over the 

 muscles which rotate the eyeballs, change the visual 

 focus for different distances, and alter the size of the 

 pupils. 



