THE BRAIN 113 



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 juicreased 

 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 

 the usual amount may cause a suspension of breath- 

 ing. This favors the view that carbon dioxid is the 

 normal stimulant for the respiratory center. It will 

 be convenient to postpone a fuller treatment of this 

 subject until the physiology of respiration is taken up. 



Other services of the medulla are connected with the 

 regulation of the circulation. It will be recalled that 

 the vagus nerves spring from the medulla and these 

 nerves contain many fibers which extend to and from 

 the heart. Destruction of the medulla will not stop 

 the beating of the heart as it stops the breathing but 



