CIRCULATION. 139 



from having three points, tricuspid ; on the left side, from having 

 two points, bicuspid. Again, those at the mouths of the pulmonary 

 artery and aorta, are, from their shape, called semilunar valves. 



The blue color at once gives an idea of the parts of the body in 

 which impure or venous blood is circulated (the veins, right side of 

 the heart and pulmonary artery). The red color also indicates in 

 what part the blood becomes purified (the lungs), and where it is 

 circulated as arterial or pure blood (the pulmonary veins, left side 

 of the heart, and arteries). It is in the capillaries between the 

 arteries and veins (which are too minute to be fully represented in 

 the figure, but which pervade every point in our bodies) that the 

 blood parts with its vivifying qualities. (See plate.) The commu- 

 nication of these qualities to the different structures, may be said, 

 indeed, to constitute the great object of the circulation. The 

 arrows in the figure show the course pursued by the blood. The 

 figure shows the situation of the heart in the chest, with the aorta 

 and its branches going off from it. The two ventricles are seen 

 united. The two auricles are also seen. The carotid arteries going 

 to the head. The superior vena cava (which empties itself into the 

 right auricle), in which the jugular veins from the brain, and the 

 subclavian veins from the arms, are seen to terminate. The water- 

 works of our city represent the circulation of the blood the reser- 

 voir the heart, the pipes the arteries. 



The course of the circulation, as shown in the figure, will now be 

 easily understood. The two vense cavse empty themselves into the 

 right auricle. From this the blood passes into the right ventricle, 

 which sends it into the pulmonary artery, and this immediately 

 divides in the lungs into innumerable branches, only a few of which 

 are represented in the figure. In the lungs, as already mentioned, 

 the blood is purified, and the pulmonary veins are therefore repre- 

 sented as bringing back to the heart red or arterial blood. The 

 pulmonary veins from both sides pour their supplies of blood into 

 the left auricle, from which again it passes into the left ventricle, 

 and is by the latter sent to all parts of the body through the aorta 

 and its branches, to be again conveyed by the veins to the right side 

 of the heart. By attending to the directions indicated by the arrows, 

 N the course of the circulation in this and the other figures will be 

 made very plain. 



To recapitulate what has just been said, on the route pursued by 

 the blood, in the apparatus of the circulation in mammiferous ani- 

 mals and birds, we see : 



