170 HAND-BOOK OF PHYSIOLOGY. 



Agents concerned in the circulation. Before quitting this sub- 

 ject it will be as well to bring together in a tabular form the various 

 agencies concerned in maintaining the circulation. 



1. The Systole and Diastole of the Heart, the former pumping into 

 the aorta and so into the arterial system a certain amount of blood, and 

 the latter to some extent sucking in the blood from the veins. 



2. The elastic and muscular coats of the arteries, which serve to keep 

 up an equable and continuous stream. 



3. The so-called vital capillary force. 



4. The pressure of the muscles on veins ivith valves, and the slight 

 rhythmic contraction of the veins. 



5. Aspiration of the Thorax during inspiration, by means of which 

 the blood is drawn from the large veins into the thorax (to be treated of 

 in next Chapter). 



DlSCOYEKY OF THE CIRCULATION. 



Up to nearly the close of the sixteenth century it was generally be-* 

 lieved that the blood passed from one ventricle to the other through fora- 

 mina in the "septum ventriculorum." These foramina are of course 

 purely imaginary, but no one ventured to dispute their existence till Ser- 

 vetus boldly stated that he could not succeed in finding them. He fur- 

 ther asserted that the blood passed from the Right to the Left side of the 

 heart by way of the lungs, and also advanced the hypothesis that it is thus 

 "revivified," remarking that the Pulmonary Artery is too large to serve 

 merely for the nutrition of the lungs (a theory then generally accepted). 



Realdus, Columbo, and Caesalpinus added several important observa- 

 tions. The latter showed that the blood is slightly cooled by passing 

 through the lungs, also that the veins swell up on the distal side of a liga- 

 ture. The existence of valves in the veins had previously been discovered 

 by Fabricius of Aquapendente, the teacher of Harvey. 



The honor of first demonstrating the general course of the circulation 

 belongs by right to Harvey, who made his grand discovery about 1618. 

 He was the first to establish the muscular structure of the heart, which 

 had been denied by many of his predecessors; and by careful study of its 

 action both in the body and when excised, ascertained the order of con- 

 traction of its cavities. He did not content himself with inferences from 

 the anatomy of the parts, bat employed the experimental method of 

 injection, and made an extensive and accurate series of observations on 

 the circulation in cold-blooded animals. He forced water through the 

 Pulmonary Artery till it trickled out through the Left Ventricle, the tip 

 of which had been cut off. Another of his experiments was to fill the 

 Right side of the heart with water, tie the Pulmonary Artery and the 

 Venae Cavae and then squeeze the Right ventricle: not a drop could be 

 forced through into the Left ventricle, and thus he conclusively disproved 

 the existence of foramina in the septum ventriculorum. "I have suffi- 

 ciently proved," says he, "that by the beating of the heart the blood 

 passes from the veins into the arteries through the ventricles, and is dis- 

 tributed over the whole body." 



"In the warmer animals, such as man, the blood passes from the Right 



