204 K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 



Mammalia differ from other pulmonated Vertebrata in the posi- 

 tion of their descending aorta. I believe that this difference 

 depends upon the greater need of blood demanded by their 

 brain. Let us consider, in some indifferent vertebrate animal, 

 the stream of blood passing from the left ventricle to the right 

 side. If the brain have a great demand for blood, this common 

 stream of blood, besides its direction to the right side, will take 

 at the same time a more forward direction than in animals with 

 a small brain and head. In the latter, therefore, the blood which 

 is intended for the posterior half of the body, after giving off the 

 blood to the anterior half, is by the influence of the current im- 

 mediately bent backwards, though continued for a little to the 

 right side. If, however, the current forward be more powerful, 

 the backward stream can only gradually overcome it, and the 

 aorta needs to pass further forwards, and to the right side ; or if 

 the influence of symmetry will not allow of this, it must bend 

 round to the left in order to pass backwards. That the blood 

 in the Mammalia actually has a more powerful current forwards, 

 is indicated by the greater length of the common trunk ; and the 

 fact that among the Mammalia again, the longer this common 

 trunk is the wider the arch, is perhaps a confirmation of what 

 has been said. In this way we may conceive the origin of the 

 difference in a formless mass. How it is developed out of the 

 symmetrical branchial apparatus of the Mammalia remains to be 

 decided by future investigations. 



Even when the heart lies in the median plane of the body, the 

 origin of the aorta, as soon as there are two ventricles, is always 

 so disposed that the impulse which is given to the blood by the 

 left ventricle passes to the right side. When the point of the heart 

 turns to the left, this relation is still more marked (see figure on 

 p. 238, arrow 1). A direction of the heart towards the right 

 side never appears to be normal. I believed that I had at times 

 observed it, but I persuaded myself that this position arose only 

 from a slipping about in the body of the animal inverted for the 

 purpose of examination. We may therefore be permitted to 

 place the heart in such a manner in our diagram as it is disposed 

 in Man and some other animals*. I choose the oblique position 



* It would seem that the symmetrical position of the heart is too generally 

 ascribed to quadrupeds. It appears to me to occur only in animals with a 



