CIRCULATION OF THE BLOOD 139 



the contrary of this does not hold good: where there is a 

 left there is not always a right ventricle. The left ven- 

 tricle I call that which is distinct in office, not in place from 

 the other, that one, namely, which distributes the blood to 

 the body at large, not to the lungs only. Hence the left 

 ventricle seems to form the principle part of the heart; 

 situated in the middle, more strongly marked, and con* 

 structed with greater care, the heart seems formed for the 

 sake of the left ventricle, and the right but to minister to it 

 The right neither reaches to the apex of the heart nor 

 is it nearly of such strength, being three times thinner in 

 its walls, and in some sort jointed on to the left (as Aris- 

 totle says), though, indeed, it is of greater capacity, in- 

 asmuch as it has not only to supply material to the left 

 ventricle, but likewise to furnish aliment to the lungs. 



It is to be observed, however, that all this is otherwise in 

 the embryo, where there is not such a difference between the 

 two ventricles. There, as in a double nut, they are nearly 

 equal in all respects, the apex of the right reaching to the 

 apex of the left, so that the heart presents itself as a sort 

 of double-pointed cone. And this is so, because in the foetus, 

 as already said, whilst the blood is not passing through the 

 lungs from the right to the left cavities of the heart, it flows 

 by the foramen ovale and ductus arteriosus directly from 

 the vena cava into the aorta, whence it is distributed to the 

 whole body. Both ventricles have, therefore, the same office 

 to perform, whence their equality of constitution. It is only 

 when the lungs come to be used and it is requisite that the 

 passages indicated should be blocked up that the difference 

 in point of strength and other things between the two ventri- 

 cles begins to be apparent. In the altered circumstances the 

 right has only to drive the blood through the lungs, whilst 

 the left has to propel it through the whole body. 



There are, moreover, within the heart numerous braces, 

 in the form of fleshy columns and fibrous bands, which 

 Ari-fotle, in his third book on "Respiration," and the "Parts 

 of Animals," entitles nerves. These are variously extended, 

 and are either distinct or contained in grooves in the walls 

 and partition, where they occasion numerous pits or depres- 

 sions. They constitute a kind of small muscles, which are 



