432 VETERINARY PHYSIOLOGY 



B. Veins. — In the veins, double flaps of the tunica 

 intima form valves which prevent any back -flow of blood. 

 The walls of the veins are thin. 



2. PHYSIOLOGY. 



The circulation of blood in the vessels is that of a fluid 

 in a closed system of elastic-walled tubes, at one end of 

 which (the great arteries) a high pressure, and at the other 

 (the great veins) a low pressure, is kept up. As a result of 

 this distribution of pressure, there is a constant flow of blood 

 from arteries to veins. 



Many points in connection with the circulation may be 

 conveniently studied on a model, or schema, made of 

 india-rubber tubes and a Higginson's syringe (Practical 

 Physiology). 



A. Blood Pressure. 



The distribution of pressure is the cause of the flow of 

 blood, and must first be considered. 



1. General Distribution of Pressure. 



That the pressure throughout the greater part of the 

 blood-vessels is positive — greater than the pressure of the 

 atmosphere — is indicated by the fact that if a vessel is 

 opened, the blood flows out of it. The force with which 

 blood escapes is a measure of the pressure in that particular 

 vessel. If an artery be cut, the blood escapes with great 

 force ; if a vein be cut, with much less force. 



1. Arteries. — If the pressure in the aorta, in the radial, 

 in the dorsalis pedis, and in one of the smallest arteries is 

 measured, it is found that there is no marked change till the 

 very smallest arteries are reached, when the pressure rapidly 

 falls. In the aorta the pressure may be over 150 mm. Hg, 

 while in the capillaries it may be only about 20 mm. Hg. 

 This distribution of arterial pressure may be plotted as in 

 fig. 183, Ar. 



2. Veins. — If the pressure in any of the small veins, in 



