K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 203 



with lungs, however, the stream of the blood passes obviously- 

 more to the right than to the left side ; so that for the nutrition 

 of the left half of the head, the blood must often come over from 

 the right side. This blood then has a motion from right to left, 

 which we must regard as a consequence of symmetry in the 

 animal part. It is unquestionably the left ventricle which is the 

 chief agent in propelling the blood, and this always sends the 

 blood to the right side. In the Lizards, Snakes, and Chelonia, 

 all the blood which is intended for the nutrition of the anterior 

 half of the body passes first in a single stream to the right 

 side, and then becomes distributed ; the blood for the anterior 

 extremity now passes forwards almost symmetrically in two 

 streams ; the blood intended for the posterior extremity in Birds 

 continues its course without dividing to the right side, and by 

 degrees, as the influence of symmetry gradually weakens that of 

 the motor power, it turns leftward again, so as to reach the me- 

 dian line. In the Amphibia above mentioned, the power of 

 symmetry sooner becomes manifest; the current of blood for 

 the posterior half of the body is also divided at the first, but 

 much more blood passes to the right than to the left side, and 

 the latter becomes sooner divided, as if it were propelled with 

 less force. In the Batrachia, the distribution is from the first 

 tolerably symmetrical. In Mammalia, the aorta descends in- 

 deed upon the left side of the vertebral column ; but if we con- 

 sider that the position of the aorta is always determined by a 

 large and common trunk for the anterior and posterior arteries, 

 that this common trunk is always directed towards the right 

 side, and first becomes turned to the left after it has given off 

 the arteries of the right anterior half of the body, or gives them 

 off as it turns ; the passage to the left side may be regarded as 

 arising from the influence of symmetry, for the arch which the 

 aorta forms is the wider the shorter the neck. In other words, 

 the aorta passes the more to the left side, the nearer it stands to 

 the head whose influence tends towards symmetry. In the 

 long-necked Mammals, where from the common trunk upon the 

 right side, that for the carotid artery and subclavian arteries (the 

 anterior aorta) passes, the branch which passes backwards turns 

 so suddenly that it can hardly be said that it passes to the left. 

 Such considerations may perhaps explain why it is that the 



