CIRCULATION OF THE BLOOD 141 



that are endowed with particularly strong frames, and with 

 large and fleshy limbs at a great distance from the heart, 

 have this central organ of greater thickness, strength, and 

 muscularity. This is manifest and necessary. Those, on the 

 contrary, that are of softer and more slender make have the 

 heart more flaccid, softer, and internally either less or not 

 at all fibrous. Consider, farther, the use of the several 

 valves, which are all so arranged that the blood, once receiv- 

 ed into the ventricles of the heart, shall never regurgitate; 

 once forced into the pulmonary artery and aorta, shall not 

 flow back upon the ventricles. 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 venae cavae and pulmonary veins, 

 lest the blood when most forcibly impelled should flow back. 

 It is for this reason that they are not found in all animals, nor 

 do they appear to have been constructed with equal care in 

 all animals in which they are found. In some they are more 

 accurately fitted, in others more remissly or carelessly con- 

 trived, and always with a view to their being closed under a 

 greater or a slighter force of the ventricle. In the left ven- 

 tricle, therefore, 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, and that the blood may not regurgitate 

 upon the pulmonary veins, and thus the force of the ventricle 

 in propelling the blood through the system at large come to 

 be neutralized, it is that these mitral valves excel those of the 

 right ventricle in size and strength and exactness of closing. 

 Hence it is essential that there can be no heart without a 

 ventricle, since this must be the source and store-house of 

 the blood. The same law does not hold good in reference 

 to the brain. For almost no genus of birds has a ventricle 

 in the brain, as is obvious in the goose and swan, the brains 

 of which nearly equal that of a rabbit in size; now rabbits 



