THE ARTERIES, 125 



motor: these are found in the sympathetic trunks (p. 215), 

 and their action may be illustrated by dividing the sympa- 

 thetic nerve of a rabbit in the neck, when immediately 

 the ear of the side experimented on gets red, and the whole 

 of that side of the head becomes warmer than the other; 

 the reason being that the paralysed arterioles no longer 

 resist the entrance of the red and warm blood, but allow it 

 to distend them. This condition is the same as takes place 

 in blushing , only in blushing the withdrawal of the nervous 

 stimulus is temporary, caused by the communication of a f 

 disturbing influence from the brain, the result of emotion. 



91. The pulse in the arteries is caused by their distension 

 and elongation under the pressure exerted by the rush of blood 

 with each beat of the heart, but can only be felt in those 

 positions in which an artery lies near some firm structure 

 against which it can be pressed. With the aid of instru- 

 ments it can be shown that it is communicated with great 

 rapidity to the whole arterial system, the smallest arteries 

 pulsating within a sixth of a second after the largest. The 

 blood already in the arteries is pushed on by each new quan- 

 tity thrown in from the hearty the velocity with which the 

 blood travels being far less than that with which the pulse 

 is communicated. The actual rate of movement of the blood 

 can be observed in the arteries of animals by the insertion 

 of an instrument for the purpose between the ends of a 

 divided vessel, and it is calculated from such experiments, 

 that in man the mean velocity is about ten inches per second 

 in the carotid, and about two and a quarter in the foot, the 

 rate of flow being much slower in small arteries than in 

 large. The reason why it is slower is to be found in a pecu- 

 liarity in the branching of arteries; for in almost all instances 

 in which an artery divides, the united areas of the divisions 

 are greater than the area of the parent trunk; and, conse- 

 quently, the total area of the combined arterial channels in- 

 creases rapidly the farther the distance from the heart. 



While the frequency of the pulse corresponds with that of 

 the heart's contractions, its character depends on a variety of 

 circumstances, of which the chief are the amount of blood 

 in the body, the vigour and regularity of the heart's action, 

 the degree to which the muscular fibres of the arterial 



