496 HANDBOOK OF PHYSIOLOGY. 



2. As a nerve-centre by which impressions are reflected, the medulla 

 oblongata also resembles the spinal cord; the only difference between 

 them consisting of the fact that many of the reflex actions performed by 

 the former are much more important to life than any performed by the 

 spinal cord. 



Demonstration of Functions. It has been proved by repeated experi- 

 ments on the lower animals that the entire brain may be gradually cut 

 away in successive portions, and yet life may continue for a considerable 

 time, and the respiratory movements be uninterrupted. Life may also 

 continue when the spinal cord is cut away in successive portions from 

 below upwards as high as the point of origin of the phrenic nerve. In 

 Amphibia, the brain has been all removed from above, and the cord, as 

 far as the medulla oblongata, from below; and so long as the medulla 

 oblongata was intact, respiration and life were maintained. But if. in 

 any animal, the medulla oblongata is wounded, particularly if it is wounded 

 in its central part, opposite the origin of the pneumogastric nerves, the 

 respiratory movements cease, and the animal dies asphyxiated. And this 

 effect ensues even when all parts of the nervous system, except the 

 medulla oblongata, are left intact 



Injury and disease in men prove the same as these experiments on 

 animals. Numerous instances are recorded in which injury to the 

 medulla oblongata has produced instantaneous death; and, indeed, it is 

 through injury of it. or of the part of the cord connecting it with the 

 origin of the phrenic nerve, that death is commonly produced in frac- 

 tures attended by sudden displacement of the upper cervical vertebrae. 



Special Centres. 



In the medulla are contained a considerable number of centres which 

 preside over many important and complicated co-ordinated movements 

 of muscles. The majority of these centres are (a.) reflex centres sim- 

 ply, which are stimulated by afferent or by voluntary impressions. 

 Some of them are (b.) automatic centres, being capable of sending 

 out efferent impulses, generally rhythmical, without previous stimu- 

 lation by afferent or by voluntary impressions. The automatic centres 

 are, however, generally influenced by reflex or by voluntary impulses- 

 Some again of the centres, whether reflex or automatic, are (c.) control 

 centres, by which subsidiary spinal centres are governed. Finally the 

 action of some of the centres is (d.) tonic, i. e., they exercise their influ- 

 ence, either directly or through another apparatus, continuously and 

 uninterruptedly in maintaining a regular action. 



(a.) Simple Reflex centres, 



(I.) A centre for the co-ordinated movements of Mastication, the 

 afferent and efferent nerves of which have been already enumerated (p. 

 230). 



