594 HANDBOOK OF PHYSIOLOGY. 



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 upward 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 oblon- 

 gata, from below; and so long as the medulla oblongata was intact, 

 respiration and life were maintained. But if, in any animal, the me- 

 dulla oblongata is wounded, particularly if it is wounded in its central 

 part, opposite the origin of the vagi, 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 me- 

 dulla 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. Th^ majority of these centres are (a.) reflex centres simply, 

 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 stimulation by afferent 

 or by voluntary impressions. The automatic centres are, however, gen- 

 erally 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 influence either directly or 

 through another apparatus, continuously and uninterruptedly in main- 

 taining a regular action. 



Simple Reflex centres. 



(1.) Bilateral centres for the co-ordinated movements of Mastication, 

 the afferent and efferent nerves of which have been already enumerated 

 (p. 326). 



(2.) Bilateral centres for the movements of Deglutition. The medulla 

 oblongata appears to contain the centre whence are derived the motor 

 impulses enabling the muscles of the palate, pharynx, and oesophagus to 

 produce the successive co-ordinate and adapted movements necessary to 

 the act of deglutition (p. 353). This is proved by the persistence of 

 swallowing in some of the lower animals after destruction of the cerebral 



