284 HANDBOOK OF PHYSIOLOGY. 



The superior laryngeal nerves are true expiratory nerves, and may 

 oe set in action when the mucous membrane of the larynx is irritated. 

 They are not constantly in action like the vagi. 



(c) Action of the glosso-pharyngeal nerves. It has been as- 

 certained, chiefly by the researches of Marckwald, that while division of 

 the glosso-pharyngeal nerves produces no effect upon respiration, stim- 

 ulation of them causes inhibition of inspiration for a short period. This 

 action accounts for the very necessary cessation of breathing during 

 swallowing. The effect of the stimulation is only temporary, and is 

 followed by normal breathing movements. 



(d) Action of other sensory nerves. The respiratory centres 

 are as a rule stimulated to produce respiration by impressions conveyed 

 by sensory nerves, e.g., the nerves of the skin; cold water applied to 

 the surface is almost invariably followed by a deep inspiration. Stimu- 

 lation of the splanchnics and of the abdominal branches of the vagi 

 produce expiration. The fifth nerves, as well as the glosso-pharyngeal 

 and the superior laryngeal, inhibit inspiration, but they tend to produce 

 a gradual slowing and not an absolute inhibition, as do the glosso- 

 pharyngeal. 



It must be remembered that although many sensory nerves may on 

 stimulation be made to produce an effect upon the respiratory centres, 

 there is no evidence to show that any one of them, except the vagi, is 

 constantly in action. The vagi indeed are, as far as we know, the only 

 normal regulators of respiration. 



Automatic Action of the Respiratory Centres. Although it has been 

 very definitely proved that the respiratory centres may be affected by 

 afferent stimuli, and particularly by those reaching them through the 

 vagi, there is reason for believing that the centres are capable of sending 

 out efferent impulses to the respiratory muscles without the action of 

 any afferent stimuli. Thus, if the brain be removed above the bulb, 

 respiration continues. If the spinal cord be divided below the bulb, the 

 facial and laryngeal respiratory movements continue, although no affer- 

 ent impulses can reach the centres except through the cranial sensory 

 nerves, and these, as we have seen, are not always in action, and indeed 

 may be divided without producing any effect, when the bulb and cord 

 are intact. As has been shown, too, respiration continues when the vagi 

 are divided. All of these experiments render it highly probable that 

 afferent impulses are not required in order that the respiratory centres 

 should send out efferent impulses of some kind to the respiratory mus- 

 cles; these centres, then, are automatic. How they act in the absence 

 of afferent stimuli has been demonstrated by Marckwald. He has shown 

 (a) firstly, that if the bulb be separated from the brain, and the vagi 

 be then cut, there is first of all inspiratory spasm followed by irregular 

 spasm of muscles both of inspiration and expiration, and death; (b) 



