COURSE OF THE BLOOD IN ANIMALS. 



61 



But there are other circumstances influencing the motion 

 of the blood in the veins. These vessels, and more especially 

 those of the limbs, are provided with valves (b), permitting 

 the blood to flow towards the heart, but preventing its reflux 

 toward the capillaries. Every intermittent compression of 

 these vessels contributes to the return of the blood to the 

 heart. 



105. The dilatation of the 



chest caused by respiration faci- 

 litates the return of the blood 

 towards the heart. By expira- 

 tion the movement of the blood 

 in the veins is momentarily in- 

 terrupted ; and the double motion 

 observed in the brain, when a 

 portion of the skull-cap has been 

 removed, is due partly to respi- 

 ration, partly to the resistance 

 which the base of the brain 

 offers to the dilatation of the 

 arteries situated between the base 

 and the skull. 



106. The course of the 

 venous blood, and the mecha- 

 nism of the cavities through which it passes from the 

 veins to the lungs, by means of the right auricle, ventricle, 

 and pulmonary artery, has been already described. In the 

 capillaries of the pulmonary artery the blood is changed into 

 arterial, and so returns by means of the pulmonary veins to 

 the left auricle of the heart. 



Course of the Blood in Different Animals. 



107. Mammals and Birds. The circulation in these 

 two classes of animals is the same (47). It is what is called 

 a double circulation, the blood passing through two sets of 

 capillaries one belonging to the body, the other to the lungs; 

 the former serving for the nutrition of the body, the latter 

 for the aeration of the blood. This kind of circulation has 

 also been called complete, which means that the whole of the 

 blood circulates through the lungs before being restored to 

 the body. 



Before birth there exists an opening between the right and 



Fig. 46. Vein, laid open. 



