CH. XXIV.] NERVOUS MECHANISM OF RESPIRATION 361 



respiratory centres in the spinal cord has been mooted, but the 

 balance of experimental evidence is against their existence. Flourens 

 found that when the respiratory centre is destroyed, respiration at 

 once ceases, and the animal dies. He therefore called it the " vital 

 knot " (noeud vitale). 



The centre is affected not only by the afferent impulses which 

 reach it from the vagus, but also by those from the cerebrum ; so that 

 we have a limited amount of voluntary control over the respiratory 

 movements. * 



The sensory nerves of the skin have also an effect. The action of 

 the air on the body of a new-born child is no doubt the principal 

 afferent cause of the first respirations. During foetal life, the need of 

 the embryo for oxygen is very small, and is amply met by the trans- 

 ference of oxygen from the maternal blood through the thin walls of 

 the foetal capillaries in the placenta. The application of cold water 

 to the skin always causes a deep inspiration ; this is another instance 

 of the reflex effect which follows stimulation of the cutaneous nerves. 

 Stimulation of the central end of the splanchnics causes expiration. 

 Stimulation of the central end of the glosso-pharyngeal causes an 

 inhibition of the respiratory movements for a short period ; this 

 accounts for the very necessary cessation of breathing during swallow- 

 ing. Stimulation of the central end of the cut superior laryngeal 

 nerve, or of its terminations in the mucous membrane of the larynx, 

 as when a crumb is " swallowed the wrong way," produces inhibition 

 of inspiratory and increase of expiratory efforts, culminating in 

 coughing. 



These nerves, however, are none of them in constant action as the 

 vagi are, and the influence of the vagus is somewhat complicated. 

 Still, respiration continues after the vagi are cut. The character of 

 the respiration becomes altered, especially if both nerves are severed ; 

 it is slower and deeper. This is due to the cessation of the impulses 

 that normally run up the vagi to the respiratory centre. The animal, 

 however, lives a considerable time ; a warm-blooded animal usually 

 dies after about a week or ten days from vagus pneumonia, due to the 

 removal of trophic influences from the lungs. Cold-blooded animals 

 live longer; they exhibit fatty degeneration of the heart-muscle also. 



The question has been much debated whether the activity of the 

 respiratory centre is automatic or reflex ; that is to say, whether the 

 rhythmic discharges proceeding from it depend on local changes 

 induced by the condition of its blood supply, or on the repeated 

 stimulations it receives by afferent nerves. 



There appears every reason to believe that the centre has the 

 power of automatism, but this is never excited under normal 

 circumstances. Normally, the respiratory process is a series of 

 reflex actions. 



