AUTOMATIC ACTION OF THE RESPIRATORY CENTERS 275 



fluenced strongly by afferent nerve impulses having their origin in general 

 sensory nerves, particularly the nerves of the skin. Cold water suddenly 

 applied to the surface of the skin is almost invariably followed by a deep 

 inspiration. Stimulation of the splanchnics and of the abdominal branches 

 of the vagi produces expiration. Stimulation of the isolated sciatic nerve of 

 the dog or the rabbit causes a marked acceleration both of the rate and 

 the amplitude of the respiratory movements, see figure 246. This accelera- 

 tion is due to afferent impulses which reach the respiratory center in the me- 

 dulla over sensory paths, paths which are not necessarily special respiratory 

 afferent paths, but rather are general afferent paths which affect the respira- 

 tory center through their numerous collaterals in the brain stem. 



It must be remembered that, although on stimulation many sensory nerves 

 may be made to produce an effect upon the respiratory centers, 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 normal regulators of respiratory 

 movements, yet one must remember that it is possible reflexly to influence 

 the respiration rate and depth through reflexes that may have their sensory 

 origin in any part of the body. 



The respiratory center is also influenced by nerve activity of the cerebral 

 cortex, psychic activity. This is illustrated by the limited voluntary control 

 of the respiration movements. 



Automatic Action of the Respiratory Centers. Although it has 

 been very definitely proved that the respiratory centers may be affected by 

 afferent stimuli, and particularly by those reaching them through the vagi, 

 there is reason for believing that the center is 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 immediately below the bulb, the facial and laryngeal 

 respiratory movements continue, although no afferent impulses can reach 

 the center except through the cranial sensory nerves, and these 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. 

 Isolation of the respiratory center from its sensory relations does not destroy 

 respiratory movements so long as the motor paths through the phrenic nerve 

 are intact. All of these experiments render it highly probable that afferent 

 impulses are not required in order that the respiratory centers should send 

 out efferent impulses to the respiratory muscles. The center, then, is auto- 

 matic. 



Method of Automatic Stimulation of the Respiratory Center. The 

 respiratory center is capable of working automatically apart from afferent 

 impulses, and this fact has been explained by the supposition that it is stimu- 

 lated to action by the condition of the blood circulating through it. When 

 the blood becomes more and more venous the action of the center becomes 



