THE BLOOD 119 



veins, thin walled vessels, which carry the 

 blood back to the heart. The smaller veins, 

 by their confluence, form larger and larger 

 veins, and the large veins, in various situations, 

 are furnished with valves which, when open, 

 are directed towards the heart, and thus direct 

 the flow of blood to that organ. The heart 

 itself, in man, has four cavities, two auricles, 

 a right and left, that receive blood, and 

 two ventricles, right and left, that drive 

 the blood out. The right auricle receives 

 the blood from the peripheral parts of the 

 body, and the left receives it from the 

 lungs. The two auricles contract simul- 

 taneously. The two ventricles then simul- 

 taneously contract, the right driving the 

 blood through the pulmonary circulation 

 arteries, pulmonary capillaries, veins to the 

 lungs, for respiratory purposes, while the left 

 ventricle drives the blood through the 

 systemic circulation, arteries, capillaries, and 

 veins, through the body, so as to bring the 

 highly oxygenated and nutritious blood to 

 the tissues. Valves are placed at various 

 orifices of the heart and they so work 



