THE REGULATING MECHANISM 187 



of the sympathetic in the neck causes it to beat 

 slightly faster, but stimulation of the vagus, 

 also in the neck, causes the heart to beat more 

 slowly ; stronger stimulation may stop the heart 

 altogether, and it will then be found that it is 

 arrested with all its cavities dilated, that is 

 to say, the muscle substance is at rest. This 

 action of the vagus is said to be inhibitory or 

 restraining, while that of the sympathetic is 

 accelerating. 



105. Such inhibitory phenomena have been 

 found in connection with many nerve centres. 

 For example the sympathetic is the nerve 

 that acts on the muscular walls of small 

 vessels, keeping them in a state of partial 

 contraction, and thus, as already explained, 

 maintaining a high blood pressure. If this 

 pressure rose too high, the heart would have 

 more work to do in driving the blood onward 

 with increased resistance. The centre (vaso- 

 motor), which thus acts through the sympa- 

 thetic, is in the medulla, and it is assumed that 

 impulses are constantly passing from it to the 

 vessels. This centre, however, may be inhib- 

 ited by the action of a nerve passing from 



