THE CONTROL OF THE RESPIRATION 331 



nature of this control and the remarkable sensitivity of the center towards 

 it have been thoroughly established. We shall return to this important 

 subject later. Meanwhile we shall proceed to examine the manner in 

 which the center is affected by sensory impulses transmitted to it. 



The afferent nerve fibers going to the respiratory center may conven- 

 iently be divided into two groups: those which act on it only occasionally, 

 and those which act on it more or less continuously. 



Th6 Occasionally Acting Impulses 



To the first group belong afferent nerves from practically every part of 

 the body. That impressions from the skin affect the respiratory center 

 is well known by the increased breathing caused by applications of cold 

 water. The influence of these afferent impulses is often very marked, 

 and is frequently taken advantage of in stimulating a newborn infant to 

 take the first breath. Stimulation of the terminations of the fifth nerve 

 in the mucous membrane of the nose, as by inhaling a pungent odor, 

 immediately inhibits respiration. To these occasionally acting afferent 

 impulses may be added the impulses that are conveyed to the respiratory 

 center from the higher nerve centers of the cerebrum. These impulses 

 are largely voluntary in nature, and enable us to hold our breath at will. 

 Some of the cerebral impulses are however also involuntary, their exist- 

 ence being seen by observing the respirations of an animal before and 

 after sectioning the pons or peduncles. The respirations for a time at least 

 become distinctly affected, but they later return with perfect regularity. 

 They may become very irregular, however, if the vagi as well as the pons 

 are cut. Other experimental evidence of the existence of cerebral respir- 

 atory fibers is furnished by cerebral localization experiments. During 

 stimulation of the cerebral cortex, for example, a marked effect on the 

 respiratory movements is often noted. 



Respiratory rhythm, unlike that of the heart, has often to be modified 

 in order that the respiratory mechanism may be used for other purposes 

 than the ventilation of the lungs. This alteration in rhythm may take 

 the form of a mere inhibition, such as the act of swallowing; or the 

 respiration may be altered, as in phonation and singing. More consid- 

 erable alteration in the expiratory discharge occurs in coughing and- 

 sneezing, and still more in the acts of micturition, defecation, and parturi- 

 tion. We must conclude therefore that the rhythmic stimuli sent out 

 from the respiratory center are so weak that stimuli from other sources 

 may instantly inhibit or change their form at any stage of the cycle. 



