THE BLOODVESSELS 95 



legs which result from standing are due to transudation, the 

 result of the pooling of blood under the influence of gravity, and 

 the absence of venous massage which results from muscular 

 activity. We see it also in the feet, where the absence of exer- 

 cise withdraws the needful stimulus from the circulation in the 

 laminae, and the feet become congested. 



It is impossible for a man in an upright position to remain 

 absolutely immovable. The slightest change in the pressure 

 on the limbs brings muscles into play which force the blood 

 along the veins. The same thing occurs in the horse, an animal 

 that may not lie down for days or weeks together (and some 

 never lie down). The weight imposed upon the hind-limbs is 

 never equally distributed during repose. First one leg, then the 

 other, takes the weight, and at every change in posture the 

 muscles compress the veins and force the blood along. In the 

 fore-leg these alternate changes in position are not necessary; 

 the veins are short and near the heart. 



The experiment of holding the domestic rabbit in the vertical 

 position referred to at p. 71 is a good example of the manner 

 in which the blood will pool under the influence of gravity in the 

 absence of muscular support. 



It is true that in the larger quadrupeds changes in posture are 

 of a far simpler character than in the biped, the most frequent, 

 perhaps, being that of the dependent head in grazing. This 

 imposes a column of venous blood in the head and neck, from 

 which it has to ascend against gravity. This is fully compen- 

 sated by the venous cisterns beneath the masseter muscle, which 

 at every masticatory act mechanically forces the blood into and 

 along the jugular, the valves preventing regurgitation. In the 

 days of bleeding, when jugular obliteration was not uncommon, 

 such horses could not be grazed, owing to the vertebral veins 

 being unable to compensate for the posture. 



The left ventricle is capable of driving the blood throughout 

 the entire vascular system, but it needs assistance. Under the 

 conditions of a normal circulation it drives it principally as far 

 as the arterioles, the skeletal muscles and valves in the veins 

 afford the venous system the necessary support, and as the heart 

 is approached the respiratory pump completes the emptying of 

 the veins and fills the right heart. 



Peculiarities in the Circulation through various tissues occur 

 as the result of their special function. They are observed in the 

 brain, erectile tissues, etc. The great vascularity of the brain 

 necessitates that the blood should pass to it with a degree of 

 uniformity which will insure the carrying out of its functions. 

 It must never be left without blood, or immediate unconscious- 

 ness would occur. We see this provided for in the frequent 



