THE CIRCULATORY OR VASCULAR SYSTEM 131 



up into small branches, which form a network around 

 the alveoli, or air-cells. The oxygenated blood is col- 

 lected by the pulmonary veins and carried to the left 

 auricle. 



The two auricles contract and force the blood into the 

 ventricles; the two ventricles then contract, and the 

 tricuspid and mitral valves close their orifices, the blood 

 being propelled on into the large vessels (the pulmonary 

 artery and the aorta), the semilunar valves then closing. 

 The blood cannot get from the right side of the heart to 

 the left without passing through the lungs, or lesser cir- 

 culation; neither can the blood get from the left side of the 

 heart to the right without passing around the general or 

 systemic circulation. 



The Pulmonary Veins. The pulmonary veins are 

 formed by the union of the capillaries in the lung; they 

 unite in two trunks from each lung, discharging their 

 blood into the left auricle of the heart. As has been 

 stated before, this is an instance where the veins transmit 

 arterial blood. In some instances there are three veins on 

 the right side, which terminate in a common opening in 

 the auricle. There are no valves in the pulmonary veins. 



THE BLOOD 



We have previously described the manner in which 

 the blood passes through the heart in circulating through 

 the body. It now remains for us to consider the function 

 of the blood, and to give a description of the manner in 

 which it is oxygenated and replenished with nutritive 

 matter. 



The blood is a homogeneous liquid, possessing a slight 

 degree of viscidity, with a consistency and density some- 

 what greater than water. It is especially distinguished 

 by its color, which is usually of a bright scarlet when it is 

 drawn from an artery, and of a dark red or purple, some- 



