82 A MANUAL OF VETERINARY PHYSIOLOGY 



to diminish in velocity, it would cover less than i J miles in an 

 hour. 



The velocity of the blood in the arteries furthest from the heart 

 is less than in those nearest the heart ; for example, in the above 

 table a horse with a carotid velocity of 30 to 40 cm. per second 

 had a metatarsal velocity of only 5-5 cm. per second. 



In the capillaries the velocity is very low, perhaps not more 

 than 0-5 to 1 mm. ( 3 \ to T V inch) per second. This reduc- 

 tion, as we have seen, is due to the width of the bed through 

 which the fluid is flowing. As the veins are reached and the 

 bed narrows, the velocity increases, until finally, as the heart is 

 approached, the velocity in the veins, though increasing, is much 

 less than that in the corresponding arteries. A horse with a 

 carotid velocity of 30*5 cm. (12 inches) gave a jugular velocity 

 of 2275 cm. (9 inches). It is generally considered that the 

 velocity of the blood in the large veins entering the heart is 

 about half as great as that in the aorta. 



During work the velocity in the vessels increases. The flow 

 of blood through the carotid of the horse has been observed by 

 Lortet to be five or six times greater during the time the animal 

 is feeding than when at rest. 



In speaking of blood-pressure we drew attention to the fact that 

 when the heart is more active or the arterioles contract the blood- 

 pressure rises, and that when the heart is less active or the 

 arterioles dilate the blood-pressure falls. These factors also 

 affect velocity. When the heart-beats increase in force, the 

 velocity of the blood is increased ; when they are diminished 

 the velocity is reduced. Further, it is obvious, from what has 

 been said of the influence of the width of the vascular bed, that 

 if the arterioles dilate the velocity of the blood through them is 

 increased, and if they contract it is diminished. 



In the pulmonary circulation the velocity is much greater 

 than in the systemic, only one-fifth or one-sixth of the time being 

 required for a lung circuit as compared wirri a body circuit. 



Any attempt made to estimate the velocity of the blood by 

 dividing an artery, and measuring the escape of blood from its 

 cut end in a given time, would lead to erroneous conclusions, for 

 the velocity in a closed artery and an open one are two different 

 things. In the undivided artery the peripheral resistance 

 reduces the velocity, in the divided artery the peripheral resist- 

 ance largely disappears and the velocity is five or ten times 

 greater, so that the carotid of a horse does not bleed with a 

 velocity of 16 inches per second, but nearer 160 inches per second. 

 Or, to put it in a practical way, if the carotid of the horse has a 

 sectional area of 0*2 square inch, the amount of blood passing 

 through the unwounded vessel amounts to 2 ounces per second, 



