THE BRAIN 111 



ceptions, supplied with motor nerve fibers originating 

 in the spinal cord. Hence, the cells in the medulla 

 must not be thought of as directly connected with these 

 muscles but with the lower order of nerve cells in the 

 cord. Here for the first time we have an example of 

 what is common enough in many other mechanisms, 

 a higher center presiding over a number of centers of 

 lower rank, as a colonel commands the captains in his 

 regiment. It may be added that in the respiratory 

 system the "captains" exercise no discretionary power 

 when their superior ceases to hold them to their work. 



The respiratory center is much subject to reflexes. 

 Almost any sudden shock is certain to modify the breath- 

 ing in some way. On the other hand we are not to 

 think that the taking of each breath is a reflex" act. 

 It is probably accomplished without the essential as- 

 sistance of afferent impulses. The center is said to 

 be automatic. This is a word we have used before to 

 describe cardiac and smooth muscle. As those two 

 contractile tissues tend to exhibit a rhythmic type of 

 activity in the absence of recognizable stimulation, 

 so the respiratory center seems disposed to go on in 

 the performance of its duty without external prompting. 



When we say that automatic tissues are active in 

 the absence of external stimuli we do not say that they 

 are not responding to a local or internal excitation. 

 We must suppose that they are. Chemical condi- 

 tions developing moment by moment doubtless de- 

 termine their behavior. Many facts bearing upon this 

 matter have been discovered and some of them we 

 shall have occasion to speak of in discussing the heart- 

 beat. As regards the respiratory center the most 

 significant point is its response to variations in the 

 carbon dioxid content of the blood. An increased 

 concentration of this gas in the blood carried to the 

 brain produces a prompt increase in the breathing 

 movements. 



A diminution of the carbon dioxid in blood below 



