176 HUMAN PHYSIOLOGY. 



The gray matter of the medulla is continuous with that of the cord. It 

 is arranged with much less regularity, becoming blended with the white 

 matter of the different columns, with the exception of the anterior. By the 

 separation of the posterior columns the transverse commissure is exposed, 

 forming part of the floor of the fourth ventricle; special collections of gray 

 matter are found in the posterior portions of the medulla, connected with the 

 roots of origin of different cranial nerves. 



Properties and Functions. The medulla is excitable anteriorly, and 

 sensitive posteriorly, to direct irritation. It serves 



1. As a, conductor of afferent or sensor impulses upward from the cord, 

 through the gray matter to the cerebrum. 



2. As a conductor of efferent, volitional or motor impulses from the brain to 

 the spinal cord and nerves, through its ventral pyramids. 



3. As a conductor of coordinating impulses from the spinal cord to the cere- 

 bellum, through the restiform bodies. 



As an Independent Reflex Center. The medulla oblongata contains 

 special collections of gray matter, constituting independent nerve centers 

 presiding over different functions, some of which are as follows viz. : 



1. A center which controls the movements of mastication, through afferent 

 and efferent nerves. 



2. A center reflecting impressions which- influence the secretion of saliva. 



3. A center for sucking, mastication, and deglutition, whence are derived motor 

 stimuli exciting to action and coordinating the muscles of the palate, 

 pharynx, and esophagus, necessary for the swallowing of the food. 



4. A center which coordinates the muscles concerned in the act of vomiting. 



5. A speech center, coordinating the various muscles necessary for the accom- 

 pishment of articulation through the hypoglossal, facial nerves, and the 

 second division of the fifth pair. 



6. A center for the harmonization of muscles concerned in expression, reflect- 

 ing its impulses through the facial nerve. 



7. A cardiac center, which exerts (i) an accelerator influence over the heart's 

 pulsations through nerve-fibers emerging from the thoracic portion of the 

 cord, in the ventral roots of the second and third thoracic nerves after which 

 they pass to the stellate ganglion around the cells of which they arborize, 

 new nerve fibers from these cells then pass to the heart; (2) an inhibitor 

 or retarding influence upon the action of the heart, through fibers of the 

 spinal accessory nerve running in the trunk of the vagus. The cardio- 

 inhibitor center is in a state of tonic activity and continuously sends im- 

 pulses to the heart which exert an inhibitory influence upon its action. It 



