XV] OF BLOOD VESSELS 667 



we may take the conditions which determine or help to determine 

 the manner of branching of an artery, or the angle at which its 

 branches are given of?; for, as John Hunter said*, "To keep up a 

 circulation sufficient for the part, and no more. Nature has varied 

 the angle of the origin of the arteries accordingly." The general 

 principle is that the form and arrangement of the blood-vessels is 

 such that the circulation proceeds with a minimum of effort, and 

 with a minimum of wall-surface, the latter condition leading to a 

 minimum of friction and being therefore included in the first. 

 What, then, should be the angle of branching, such that there 

 shall be the least possible loss of energy in the course of the 

 circulation? In order to solve this problem in any particular 

 case we should obviously require to know (1) how the loss of 

 energy depends upon the distance travelled, and (2) how the loss 

 of energy varies with the diameter of the vessel. The loss of 

 energy is evidently greater in a narrow tube than in a wide one, 

 and greater, obviously, in a long journey than a short. If the 

 large artery, AB, give off a comparatively 

 narrow branch leading to P (such as CP, 

 or DP), the route ACP is evidently 

 shorter than ADP, but on the other 

 hand, by the latter path, the blood has 

 tarried longer in the wide vessel AB, 

 and has had a shorter course in the 

 narrow branch. The relative advantage 

 of the two paths will depend on the loss 

 of energv in the portion CD, as com- 

 pared with that in the alternative portion 



CD' , the latter being short and narrow, the former long and wide. 

 If we ask, then, which factor is the more important, length or 

 width, we may safely take it that the question is one of degree : 

 and that the factor of width will become much the more important 

 wherever the artery and its branch are markedly unequal in size. 

 In other words, it would seem that for small branches a large 

 angle of bifurcation, and for large branches a small one, is always 

 the better. Roux has laid down certain rules in regard to the 

 branching of arteries, which correspond with the general con- 

 * Essays, etc., edited by Owen, i, p. 134, 1861. 



