COURSE OF THE BLOOD IN ANIMALS. 



57 



observed in the brain, when a portion of the skull-cap has 

 been removed, is due partly to respiration, partly to the 

 resistance which the base of the brain offers to the dilata- 

 tion of the arteries situated between the base and the skull. 



106. The course of the venous blood, and the me- 

 chanism 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 (38). 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 



Smaller Circulation. 



Pulmonary Artery. 



Right Auricle 

 Heart. 



Venae Cavae. 



Right Ventricle. 



Pulmonary Veins. 

 - Left Auricle. 



Aorta. 



Left Ventricle. 



Greater Circulation. 

 Fig. 33. Mammals and Birds. 



THEORETIC FIGURE OF THE CIBCULATIOJT. 



