246 PHYSIOLOGY AT THE FARM. 



Also the black blood of the system of the pulmonary artery, 

 in its passage through the capillaries to reach the radicles of 

 the system of the pulmonary veins, changes to red blood. 



Thus the vascular organs that is, the cavities of the heart 

 and the blood-vessels are conveniently arranged into those 

 which convey black blood ; namely, the system of the vence 

 cavce, the right cavities of the heart, and the system of the 

 pulmonary artery and those which convey red blood namely, 

 the system of the pulmonary veins, the left cavities of the 

 heart, and the system of the aorta. Each of these divisions 

 has its moving force namely, its own portion of the contrac- 

 tile heart placed in the middle ; that is to say, the right auricle 

 and the right ventricle, propelling the black blood by their 

 contraction, stand in the middle between the system of the 

 vence cavce and the system of the pulmonary artery ; while the 

 left auricle and the left ventricle, propelling the red blood, 

 stand in the middle between the system of the pulmonary 

 veins and the system of the aorta. Moreover, the points of 

 demarcation between these two divisions of the vascular ap- 

 paratus are, on the one hand, in the capillaries in which the 

 red blood, during the operations of assimilation, becomes black ; 

 and, on the other, in the capillaries in which the black blood, 

 by the agency of the atmosphere in respiration, becomes red. 



There are in the heart two chambers on the right side and 

 two chambers on the left side. The chamber which receives 

 blood coming from a distance is on each side termed an auricle ; 

 the chamber which sends forth blood to a distance is on each 

 side termed a ventricle. The vessels which bring the blood to 

 the heart on each side are named veins, those which carry the 

 blood out from the heart are known as arteries. 



When a particle of blood has found its way, for example, 

 into the left ventricle, there is only one course by which, after 

 leaving that ventricle, it can return to it again. In so return- 



