AUTOMATIC ACTION OF THE RESPIRATORY CENTERS 313 



cessation of breathing during swallowing. The effect of the stimulation is 

 only temporary, and is followed by normal breathing movements. 



Action of Other Sensory Nerves. The respiratory center is in- 

 fluenced strongly by afferent nerve impulses having their origin in general 

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

 dashed on the skin is followed by a deep inspiration. Stimulation of the 

 splanchnics or of the abdominal branches of the vagi produces expiration. 

 Stimulation of the isolated sciatic nerve of the dog or of the rabbit causes a 

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

 movements, see figure 246 b. 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 nerve 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 



