360 



OF THE CIRCULATION OF BLOOD. 



when it is stated that the unit, in the above measurements, is no more than 

 one-fortieth of an inch; and when it is remembered that any error in the 

 measurement is greatly increased in the calculation.* 



477. The ultimate ramifications of the Arteries usually pass so insensibly 

 into those of the Veins, that the line of demarkation between them cannot be 

 distinctly drawn. Hence there is no ground for the assumption that the 

 Capillaries form a distinct system of vessels, in which the Arteries terminate 

 and the Veins arise. They are in no respect different, except in size, from 

 the other vessels. Their anastomosis is very frequent, so that a minute net- 

 work is formed by them ; but this is also seen in the distribution of the larger 

 vessels. It has been maintained by some, that they are mere passages, chan- 

 neled out of the tissues through which they convey the blood ; but this, again, 

 is incorrect, for recent microscopical observations have shown, that they have 



Fig. 89. 



a A 



Capillary circulation in a portion of the web of a .Frog's foot, magnified 110 diameters; 1, trunk of vein ; 

 2, 2, 2, its branches ; 3, 3, pigment-cells. (After Wagner.) 



* From Mr. Paget's observations, it appears that there is seldom an exact equality be- 

 tween the area of the trunk and that of its branches; but the area sometimes increases, 

 and sometimes diminishes ; the former being the general rule for the subdivision of the 

 aorta and its principal branches in the upper extremities ; the latter in the lower. The 

 following Table shows the relative areas of several arterial trunks, and of the branches 

 proceeding from them. 



Trunk. 



Arch of Aorta 



Innominata - ... 



Common carotid ... 



External carotid 



Subclavian - 



Abdominal Aorta, to last lumbar art. 



, just before dividing 



Common Iliac 

 External Iliac 



Branches. 



1-055 



1-147 



1-013 



1-19 



1-055 



1-183 



893 



982 



1-15 



