614 THE NERVOUS SYSTEM 



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 which are bilateral are: i. 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 pro- 

 duce the successive co-ordinated and adapted movements necessary to the 

 act of deglutition, page 339. 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, 

 figure 400; and by the complete arrest of the power of swallowing when 

 the medulla oblongata is injured in experiments. 



2. 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. Centers for the secretion of saliva, which have been already mentioned, 

 page 349- 



4. Centers for vomiting, page 376. 



5. Centers for coughing, which is a reflex act quite independent of the 

 respiratory act. 



6. 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 cervical 

 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 im- 

 pulses. 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 cannot 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 taking place constantly in response to afferent impulses flowing 

 into the medulla from numerous other sensory nerves over the entire body. 



8. The Cardio-inhibitory Centers. The medulla contains the centers 

 which maintain the proper rhythm of the heart, these centers acting through 

 the vagus fibers. These terminate in a local intrinsic mechanism which has 



