HEART AND BLOOD. 79 



as elsewhere : as the lungs are spongy, and loose, and soft, no 

 great amount of force is required to force the blood through 

 them ; hence the right ventricle is either without the bundles 

 in question, or they are fewer and weaker, not so fleshy 

 or like muscles ; those of the left ventricle, however, are both 

 stronger and more numerous, more fleshy and muscular, because 

 the left ventricle requires to be stronger, inasmuch as the blood 

 which it propels has to be driven through the whole body. And 

 this, too, is the reason why the left ventricle occupies the middle 

 of the heart, and has parietes three times thicker and 

 stronger than those of the right. Hence all animals and among 

 men it is not otherwise that are endowed with particularly 

 strong frames, and that have large and fleshy limbs at a great 

 distance from the heart, have this central organ of greater thick- 

 ness, strength, and muscularity. And this is both obvious and 

 necessary. Those, on the contrary, that are of softer and more 

 slender make have the heart more flaccid, softer, and internally 

 either sparely or not at all fibrous. Consider farther the use of 

 the several valves, which are all so arranged, that the blood once 

 received into the ventricles of the heart shall never regurgitate, 

 once forced into the pulmonary artery and aorta shall not flow 

 backupontheventricles. When the valves are raised and brought 

 together they form a three cornered line, such as is left by the 

 bite of a leech ; and the more they are forced, the more firmly 

 do they oppose the passage of the blood. The tricuspid valves 

 are placed, like gate-keepers, at the entrance into the ventricles 

 from the vense cavae and pulmonary veins, lest the blood when 

 most forcibly impelled should flow back ; and it is for this reason 

 that they are not found in all animals ; neither do they appear 

 to have been constructed with equal care in all the animals in 

 which they are found ; in some they are more accurately fitted, 

 in others more remissly or carelessly contrived, and always 

 with a view to their being closed under a greater or a slighter 

 force of the ventricle. In the left ventricle, therefore, and in 

 order that the occlusion may be the more perfect against the 

 greater impulse, there are only two valves, like a mitre, and 

 produced into an elongated cone, so that they come together 

 and touch to their middle ; a circumstance which perhaps led 

 Aristotle into the error of supposing this ventricle to be double, 

 the division taking place transversely. For the same reason, 



