464 VETERINARY PHYSIOLOGY 



venous pressure, dilatation to raise it. In the legs the veins 

 are abundantly supplied with valves which support the long 

 column of blood. When the veins of the legs become over- 

 distended and the valves incompetent, the veins become large 

 and tortuous and are known as varicose veins. The condi- 

 tion is temporarily relieved by elevating the legs. 



Compression of the thorax retards the flow of blood from 

 the great veins into the heart, and thus tends to raise the 

 venous and to lower the arterial pressure. 



Venous pressure may be temporarily modified by the 

 loss or gain of water, but the venous system is so capacious 

 that it can accommodate a considerably increased volume of 

 fluid without any marked rise of pressure. Further, there 

 is a very rapid adjustment between the fluid in the vessels 

 and in the tissues. 



IV. Pressure in the Lymphatics. 



No exact determination of the lymph pressure in the 

 tissue spaces has been made, but, since there is a constant 

 flow from these spaces through the lymphatic vessels and 

 through the thoracic duct into the veins at the root of the 

 neck, the pressure in the tissue spaces must be higher than 

 the pressure in the great veins. 



This pressure is kept up by the formation of lymph from 

 the blood, and from the cells of the tissues (see p. 508). 



B. FLOW OF BLOOD. 



The flow of blood, as already indicated, depends upon the 

 distribution of pressure, a fluid always tending to flow from 

 the point of higher pressure to the point of lower pressure. 

 Since a high pressure is maintained in the aorta and a low 

 pressure in the veins entering the heart and in the cavities 

 of the heart during its diastole, the blood must flow through 

 the vessels from arteries to veins {Practical Physiology). If 

 for any reason the difference of pressure is decreased, the 



