CHAPTER XXVII. 



THE NERVOUS FACTORS OF THE RESPIRATORY 

 MECHANISM. ASPHYXIA. 



The Respiratory Centre. The respiratory movements 

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

 breathe faster or slower, shallower or more deeply, as 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 are quite involuntary; they go- on 

 perfectly without the least attention on our part, and, not 

 only in sleep, but during the unconsciousness of fainting or 

 of an apoplectic fit. The natural breathing 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 contract in the 

 Body under the influence of the nerves going to them; the 

 nerves of the diaphragm are the two phrenic nerves, one for 

 each side of it ; the external intercostal muscles are supplied 

 by certain branches of the thoracic 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 oblongata. 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 

 the medulla oblongata only, the rest of the brain being un- 

 developed, and yet they breathe for a time. If, on the 

 other hand, the spinal cord be divided immediately below 

 the medulla of an animal, all breathing movements of the 



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