438 THE CIRCULATION OF THE BLOOD 



there are certain other valves in the interior of the heart 

 itself. 



I have here (Fig. 1) a purposely rough, but, so far as it 

 goes, accurate, diagram of the structure of the heart and 

 the course of the blood. The heart is supposed to be 

 divided into two portions. It would be possible, by very 

 careful dissection, to split the heart down the middle of a 

 partition, or so-called septum, which exists in it, and to 

 divide it into the two portions which you see here repre- 

 sented ; in which case we should have a left heart and a 

 right heart, quite distinct from one another. You will 

 observe that there is a portion of each heart which is what 

 is called the ventricle. Now the ancients applied the term 

 heart simply and solely to the ventricles. They did not 

 count the rest of the heart what we now speak of as the 

 auricles as any part of the heart at all ; but when they 

 spoke of the heart they meant the left and the right ven- 

 tricles ; and they described those great vessels, which we 

 now call the pulmonary veins and the vena cava, as opening 

 directly into the heart itself. 



What Erasistratus made out was that, at the roots of the 

 aorta and the pulmonary artery (Fig. 1) there were valves, 

 which opened in the direction indicated by the arrows ; ' 

 and, on the other hand, that at the junction of what he 

 called the veins with the heart there were other valves, 

 which also opened again in the direction indicated by the 

 arrows. This was a very capital discovery, because it 

 proved that if the heart was full of fluid, and if there were 

 any means of causing that fluid in the ventricles to move, 

 then the fluid could move only in one direction ; for you 

 will observe that, as soon as the fluid is compressed, the 

 two valves between the ventricles and the veins will be 

 shut, and the fluid will be obliged to move into the arteries ; 

 and, if it tries to get back from them into the heart, it is 

 prevented from doing so by the valves at the origin of the 

 arteries, which we now call the semilunar valves (half-moon 

 shaped valves) ; so that it is impossible, if the fluid move 

 at all, that it should move in any other way than from the 

 great veins into the arteries. Now that was a very remark- 

 able and striking discovery. 



But it is not given to any man to be altogether right (that 

 is a reflection which it is very desirable for every man who 

 has had the good luck to be nearly right once, always to bear 

 in mind) ; and Erasistratus, while he made this capital and 



