CHAPTER XXVI. 



THE NERVOUS FACTORS OF THE RESPIRA- 

 TORY MECHANISM. ASPHYXIA. 



The Respiratory Centre. The respiratory movement? 

 are to a certain extent under the control of the will; we 

 can breathe faster or slower, shallower or more deeply, a? 

 we wish, and can also " hold the breath" for some time but 

 the voluntary control thus exerted is limited in extent: 

 no one can commit suicide by holding his breath. In 

 ordinary quiet breathing the movements arc quite involun- 

 tary; they go on perfectly without the least attention or 

 our part, and, not only in sleep, but during the unconscious- 

 ness of fainting or of an apoplectic fit. The natural breath- 

 ing movements are therefore either reflex or automatic. 



The muscles concerned in producing the changes in the 

 chest which lead to the entry or exit of air are of the 

 ordinary striped kind; and these, as we have seen, only con- 

 tract in the Body under the influence of the nerves going 

 to them; the nerves of the diaphragm are the two phrenic 

 nerves (p. 161), one for each side of it; the external inter- 

 costal muscles are supplied by certain branches of the dor- 

 sal spinal nerves, called the intercostal nerves. If the 

 phrenic nerves be cut the diaphragm ceases its contractions, 

 and a similar paralysis of the external intercostals follows 

 section of the intercostal nerves. 



Since the inspiratory muscles only act when stimulated 

 by nervous impulses reaching them, we have next to seek 

 where these impulses originate; and experiment shows that 

 it is in the medulla ollongata. All the brain of a cat or a 

 rabbit in front of the medulla can be removed, and it will 

 still go on breathing; and children are sometimes born with 



