64 



A Manual of Veterinary Physiology. 



bed of the vessels from the heart to the capillaries, and 

 the decrease in width from the capillaries to the heart, 

 account for the high blood 'pressure in the aorta, rapid 

 flow of blood and throbbing in the arteries, the slower flow 

 and lower pressure in the capillaries, and the pulseless flow 

 and low pressure in the veins. 



We have noted that the pressure of the blood is highest 

 in the arteries and least in the veins ; it is this difference in 

 blood pressure which is the cause of the circulation ; if the 

 pressure were the same throughout the whole circulatory 

 system there would be no movement of blood, for the latter 

 naturally moves from the higher to the lower pressure. 



By Blood Pressure we understand the actual pressure 



Fig. 2. — Diagram of Blood Pressure. 



a Arteries. i', Peripheral Region (minute arteries, capillaries, 

 and veins), v, Veins (Foster). 



exerted by the blood on the walls of the vessel due to the 

 action of the heart ; this pressure can be measured by 

 suitable instruments, and is found to be in the carotid of 

 the horse from 8'88 feet to 13-53 feet of blood — that is, the 

 pressure in the vessel is capable of supporting a column of 

 blood from !) to 13 feet in height. The pressure is con- 

 stantly varying, it increases with the contraction <>!' the 

 ventricle and falls with its relaxation, it increases with 

 expiration and falls at inspiration. 



As we pass from the aorta to the capillaries the Mood 

 pressure becomes gradually reduced : in the capillaries the 



