132 VARIATIONS IN VELOCITY. [BOOK i. 



elasticity into play. In this case the intermittence of the arterial 

 flow is continued on into the veins. An instance of this is seen in 

 the experiments on the sub-maxillary gland, where sometimes the 

 resistance offered by the minute arteries of the gland is so much 

 lowered, that the pulse is carried right through the capillaries, and 

 the blood in the veins of the gland pulsates 1 . A like result occurs 

 when, the peripheral resistance remaining the same, the frequency 

 of the heart's beat is lowered. Thus the beats may be so 

 infrequent that the whole quantity sent on by a stroke has time to 

 escape before the next stroke comes. Lastly, if, while the heart's 

 beat and the peripheral resistance remain the same, the arterial 

 walls become more rigid, the arteries will be unable to expand 

 sufficiently to retain the surplus of each stroke or to exert sufficient 

 elastic reaction to carry forward the stream between the strokes ; 

 land in consequence more or less intermittence will become manifest. 



Circumstances determining the velocity of the flow. We 



have seen that the velocity of the blood-stream diminishes from 

 the aorta to the capillaries, and increases from the capillaries to 

 the great veins. Thus in the dog the velocity in the great arteries 

 may be stated at from 300 to 500 mm., in the capillaries at less 

 than 1 mm. ('5 to *75 mm.), and in the large veins at about 200 mm. 

 in a sec. In fact, the greater part of the time of the circuit is 

 taken up in the capillary region. An iron salt, injected into the 

 jugular vein of one side of the neck of a horse, makes its appear- 

 ance in the blood of the jugular vein of the other side in about 

 30 seconds. 



Bering's mean result in the horse was 27'G sees. In the dog 

 Vierordt found it to be 15*2 sees.; in the rabbit 7 sees. 



Without laying too much stress on this experiment, it may be 

 taken as a fair indication of the time in which the whole circuit 

 may be completed. It takes about the same time to pass through 

 about 20 mm. of capillaries. Hence, if any corpuscle had in its 

 circuit to pass through 10 mm. of capillaries, half the whole time 

 of its journey would be spent in the narrow channels of the 



.capillaries. Since, however, the average length of a capillary 

 is about '5 mm., about one second is spent in the capillaries. In- 

 asmuch as the purposes served by the blood are chiefly carried out 

 in the capillaries, it is obviously of advantage that its stay in them 



I should be prolonged. 



The local differences in the velocity of the stream are directly 

 dependent on the area of the 'bed.' When a fluid is driven 

 by a uniform pressure through a narrow tube with an enlarge- 

 ment in the middle, the velocity of the stream diminishes in 

 the enlargement, but increases again when the tube once more 

 narrows. So a river slackens speed in a ' broad' but rushes on 



1 See Book i. cap. i. sec. 2, on the Secretion of the Digestive Juices. 



