286 VETERINARY PHYSIOLOGY 



in the vessels may take place without materially modifying 

 the arterial or capillary pressure while affecting temporarily 

 the venous pressure. 



III. PRESSUKE IN THE VEINS 



The pressure in the veins is so low that it may best be 

 determined by a water manometer. 



In the veins the force of inflow is small ; the resistance to 

 outflow is nil. Hence the pressure is small, and steadily 

 diminishes from the small veins to the large veins entering 

 the heart (fig. 131). 



The venous pressure may be modified by variations in 

 these two factors. Constriction of the arterioles tends to 

 lower the venous pressure, dilatation to raise it. On the 

 other hand, increased heart's action, which so markedly 

 tends to raise arterial pressure, diminishes the pressure in 

 the larger veins, because the blood is thus more rapidly 

 driven from veins into arteries, and because the heart, which 

 in its powerful systole drives out more blood, in its diastole 

 sucks in more. 



Compression of the thorax has a very marked effect in 

 retarding the flow of blood from the great veins into the 

 heart, and thus tends to raise the venous pressure and to 

 lower the arterial pressure. Venous pressure may be tem- 

 porarily modified by the loss or gain of water. 



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. 223). 





