THE CIRCULATION. 193 



on the descending part of the curve, is always present. 

 This presence of two crests on the pulse wave has given to 

 it the name of dichrotic. The explanation of this second 

 or smaller wave superposed on the first, is still not wholly 

 satisfactory. Various suggestions are offered by different 

 physiologists, but the probability is that the second wave is 

 due to the reflection of the pulse wave against the semi- 

 lunar valves. 



To explain this more fully let us picture the condition 

 of things just at the moment the ventricle forces its con- 

 tents into the aorta. Here, in order to make room for the 

 sudden addition, the aorta expands close to the heart, 

 which expansion then in the form of a wave proceeds from 

 that point to the periphery; but, like the waves caused by 

 a pebble in the water, they would naturally run in all di- 

 rections, and so the pulse wave would start to run back 

 towards the heart. But scarcely started in that direction, 

 it would meet the closed semilunar valves and be there re- 

 flected backwards on the heels of the original pulse wave 

 and run with it to the tips of the arteries. Just as a loud 

 sound proceeding in a certain direction might have upon 

 its heels a slight echo caused by some precipice just behind. 



THE INNEBVATION OF THE BLOOD VESSELS. 



Many experiences and observations show that blood ves- 

 sels have a nervous control separate from the control of the 

 heart. Thus, blushing with embarrassment, or turning 

 pale with fright, or becoming flushed with exercise clearly in- 

 dicate that nerves must affect the contraction and dilatation 

 of arteries. The fact that arteries have muscular tissue in 

 their walls makes such a contraction or dilatation intelligible, 

 and no doubt the nerves in question reach these muscles. 

 Experiments have not failed to disclose such nerves which, 

 when stimulated, cause the arteries either to contract or 

 expand. Being thus concerned in controlling the motion 

 of the muscles of the arteries, they are called vaso-motor 

 nerves. In their physiological effect these vaso-motor 



