COURSE OF THE BLOOD. 641 



about the base of the ventricles. In the course of the cardiac nerves, 

 certain enlargements have been described as microscopic ganglia 

 (Lee) ; but according to other anatomists, these are thickenings of the 

 sheaths of the nerves. The lining membrane of the sinus of the right 

 ventricle, is most abundantly supplied with nerves. 



Course and Causes of the Circulation. 



In moving through the human body, the blood takes the following 

 definite twofold course, the circulation in Man, like the heart itself, 

 being double. Proceeding from the left side of the heart through the 

 aorta (Fig. 108, 9, 9), and its various branches, called the systemic 

 arteries, the blood is distributed to every part of the frame, reaching 

 the capillary vessels throughout the body generally ; from these capil- 

 laries, it passes into the minute venules, and collected into larger and 

 larger veins, 1, 2, finds its way back to the right side of the heart, 3, 

 4: this part of the circulation is called the greater or systemic circula- 

 tion. From the right side of the heart, 3, 4, the blood issues through 

 the pulmonary artery, 5, and by its branches is conveved to the lungs, 

 passes through the pulmonary capillaries, is collected by the pulmon- 

 ary veins, 6, and so returns once more to the left side of the heart, 7, 

 8 : this part of the circulation, is called the lesser or pulmonary circu- 

 lation. In' the systemic circulation, therefore, the blood leaves the 

 left side of the heart, and returns to the right side ; in the pulmonary 

 circulation, the blood leaves the right side and returns to the left. 

 The left side of this organ, is sometimes called the systemic, and the 

 right side the pulmonary heart. The greater circulation being per- 

 formed through the body generally, and the lesser circulation through 

 the lungs, the two are continuous, at the heart, with each other, a 

 given portion of blood passing first through the one, then through the 

 other, afterwards through the former again, and so on. The portal 

 circulation, already described (p. 533), is a special offset of the systemic 

 circulation. 



The history of the discovery of the Circulation, affords a good illus- 

 tration of the slow and labored steps by which man arrives at true 

 knowledge. By Hippocrates, 400 B.C., the veins and arteries were 

 confounded under one name, pAe/9&, phlebes, the word artery, being 

 applied by him to the trachea or windpipe. Aristotle distinguished 

 the arteries from the veins, and noticed that the former were usually 

 found empty of blood, containing only air: the heart, however, was 

 known by him to be in connection with the veins, and these were sup- 

 posed to convey the blood into the body. Galen was the first to main- 

 tain that the arteries contained blood as well as air. Vesalius pointed 

 out that the two sides of the heart have no direct communication. 

 Servetus demonstrated the passage of the blood through the lungs. 

 The term ''circulation" first occurs in the writings of Csesalpinus, 

 who had access to a treatise by Servetus. At length, in 1628, William 

 Harvey published in his work, " De motu cordis et sanguinis" on 

 the motion of the heart and blood an account of his great discovery, 

 or demonstration of the real course of the blood, or of the Circulation. 



41 



