66 THE MECHANISM OF LIFE 



wHcli go to the head, several to the " shoulders " of the 

 fish, and one runs down the body just underneath the back- 

 bone and supplies the viscera and the muscles of the body 

 and tail. 



Thus there is a fairly simple and complete circulation in the 

 fish. The blood is propelled by the heart into and through the 

 gills, and then it is distributed through the arteries to all parts 

 of the body. Having traversed every part of the latter, it 

 returns to the heart again via the great veins. 



The reader will now easily understand the main scheme of 

 circulation in the warm-blooded animals, including man. Here 

 we have a double heart, one half of which (the right one) is 

 connected with the lungs, and the other (left) half with the rest 

 of the body. 



Tracing out the paths taken by the blood-stream, we will see 

 that the caval veins discharge into the right auricle, which con- 

 tracts and sends the blood into the right ventricle. From there 

 it goes through the pulmonary arteries into the right and left 

 lungs, and having traversed the capillaries in these organs, it is 

 returned to the left auricle by the pulmonary veins. The left 

 auricle forces it into the left ventricle, and from there it is 

 propelled all over the body, being distributed by the great aorta 

 and the arteries that branch out from the latter. 



The first complete demonstration of the circulation of the 

 blood must have appealed to physiologists as a perfect proof of 

 the mechanical conception of life. This proof, however, was 

 slow in coming, and many men contributed to it. Servetus, a 

 scholar of the early sixteenth century, seems to have discovered 

 the pulmonary circulation, and, curiously enough, he announced 

 it in 1553 in a theological work called De Restitutio Christianismi, 

 for the publication of which he was hunted out from Spain by 

 the Inquisition, only to encounter the intolerance of Calvin at 

 Geneva, where he was burned. The English physician Harvey 

 discovered the other, or systemic circulation, and gave a com- 

 plete and formal demonstration of the whole scheme in 1628. 

 Fabricius had already discovered the valves in the veins, and 

 in 1661 Malpighi found the connection betw^een arteries and 

 veins — the innumerable, minute, hairlike vessels called the 

 capillaries — and that completed the proof. But even before the 

 latter discoveries the great mind of Descartes had made use of 

 the incomplete demonstration of Harvey to establish his con- 



