300 RESPIRATION 



marked acceleration both of the rate and of the amplitude of the respiratory 

 movements, see figure 246. This acceleration is due to afferent impulses 

 which reach the respiratory center in the medulla over sensory paths, paths 

 which are not necessarily special respiratory afferent paths, but rather are 

 general afferent paths which affect the respiratory center through their 

 numerous collaterals in the brain stem. 



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

 stimulation be made to produce an effect upon the respiratory center, yet 

 there is no evidence to show that any one of them, except the vagus, is con- 

 stantly in action. The vagi indeed are, as far as we know, the normal 

 regulators of respiratory movements, yet it is possible reflexly to influence 

 the respiratory rate and depth through impulses that may have their origin 

 in any sensory 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 respiratory 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 nerves 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 automatic. 



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 

 stimulated 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 more and more energetic, and if the air is prevented from entering 

 the chest, the respiration in a short time becomes very labored. If the 

 aeration of the blood is much interfered with, not only are the ordinary 

 respiratory muscles employed, but also those muscles of extraordinary in- 

 spiration and expiration which have been previously enumerated. Thus, 



