542 THE NERVOUS SYSTEM 



death; and, indeed, it is through injury to it, or of the part of the cord con- 

 necting it with the origin of the phrenic nerve, that death is commonly pro- 

 duced in fractures attended by sudden displacement of the upper cervical 

 vertebrae. 



The majority of the medullary centers are reflex centers simply, and are 

 stimulated by afferent or by voluntary impulses. Some of them are auto- 

 matic centers and are capable of sending out efferent impulses without pre- 

 vious stimulation by afferent or by voluntary impulses. The automatic 

 centers are, however, normally influenced by reflex or by voluntary impulses. 



Some of these reflex centers are: i. Bilateral centers for the movements of 

 deglutition. The medulla oblongata contains in the motor nuclei of the ninth 

 and tenth nerves the centers whence are derived the motor impulses enabling 

 the muscles of the palate, pharynx, and esophagus to produce the successive 

 coordinated and adapted movements necessary to the act of deglutition, page 

 313. This is proved by the persistence of the act of swallowing in some of 

 the lower animals after destruction of the cerebral hemispheres and cerebellum; 

 its existence in anencephalous monsters; and by the complete arrest of the 

 power of swallowing when the medulla oblongata is injured in experiments. 



2. Bilateral centers for the combined muscular movements of sucking, the 

 nerves concerned being the facial for the lips and mouth, the hypoglossal for 

 the tongue, and the inferior maxillary division of the fifth for the muscles of the 

 jaw. 



3. Bilateral centers for the secretion 0} saliva, which have been already 

 mentioned, page 305. 



4. Bilateral centers for vomiting, page 330. 



5. Bilateral centers for coughing, which is a reflex act quite independent 

 of the respiratory act. The coughing center is situated above the inspiratory 

 part of the respiratory center. 



6. Bilateral centers for the dilatation of the pupil, the fibers from which 

 pass out through the spinal cord in the two upper dorsal nerves into the cervi- 

 cal sympathetic. 



7. The respiratory center of the medulla has already been discussed as 

 regards its automatic action. It is only necessary to repeat here that although 

 it is automatic in its action, being capable of direct discharge of respiratory 

 impulses with no other stimulus than the condition of the blood circulating 

 within it, yet it is constantly reflexly influenced by afferent impulses. The 

 respiratory center has been proven to be bilateral. It also consists of an 

 inspiratory part and of an expiratory part. The center is influenced by 

 voluntary impulses, but one can not voluntarily control this center to the 

 point of death. The vagus influence is probably the most constant of those 

 stimulating the respiratory center. But the respiratory reflexes are going on 

 constantly in response to afferent impulses flowing into the medulla from nu- 

 merous other sensory nerves over the entire body. 



