Heart and Blood 101 



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 ; 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, indeed, 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, 

 too, it is essential that there can be no heart without a 

 ventricle, since this must be the source and storehouse 

 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 have ventricles in the 

 brain, whilst the goose has none. In like manner, 

 wherever the heart has a single ventricle, there is an 

 auricle appended, flaccid, membranous, hollow, filled 

 with blood ; and where there are two ventricles, there 

 are likewise two auricles. On the other hand, however, 



