276 GENERAL ANATOMY OF ARTERIES. 



that no impediment may be offered to the free circulation of the vital fluid. 

 The division of arteries is usually dichotomous, as of the aorta into the 

 two common iliacs, common carotid into the external and internal, &c. ; 

 but in some few instances a short trunk divides suddenly into several 

 branches, which proceed in different directions ; this mode of division is 

 termed an aocis, as the thyroid and coeliac axis. 



In the division of an artery into two branches, it is observed that th 

 combined arese of the two branches are somewhat greater than that of th 

 single trunk ; and if the combined arese of all the branches at the peri 

 phery of the body were compared with that of the aorta, it would be seen 

 that the blood, in passing from the aorta into the numerous distributing 

 branches, \vas flowing through a conical space, of which the apex might 

 be represented by the aorta, and the base by the surface of the body. 

 The advantage of this important principle in facilitating the circulation is 

 sufficiently obvious ; for the increased channel which is thus provided for 

 the current of the blood, serves to compensate for the retarding influence 

 of friction, resulting from the distance of the heart and the division of the 

 vessels. 



Communications between arteries are very free and numerous, and in- 

 crease in frequency with the diminution in size of the branches ; so that, 

 through the medium of the minute ramifications, the entire body may be 

 considered as one uninterrupted circle of inosculations , or anastomoses 

 (dva between, fa^a. mouth). This increase in the frequency of anastomosis 

 in the smaller branches is a provision for counteracting the greater liability 

 to impediment existing in them than in the larger branches. Where free- 

 dom of circulation is of vital importance, this communication of the arteries 

 is very remarkable, as in the circle of Willis in the cranium, or in the dis- 

 tribution of the arteries of Ihe heart. It is also strikingly seen in situations 

 where obstruction is most likely to occur, as in the distribution to the ali- 

 mentary canal, around joints, or in the hand and foot. Upon this free 

 communication existing everywhere between arterial branches is founded 

 the principle of cure in the ligature of large arteries ; the ramifications of ' 

 the branches given off from the artery above the ligature inosculate with 

 those which proceed from the trunk of the vessel below the ligature; these 

 anastomosing branches enlarge and constitute a collateral circulation, in 

 which, as is shown in the beautifiul preparations made by Sir Astley 

 Cooper, several large branches perform the office of the single obliterated 

 trunk.* 



The arteries do not terminate directly in veins ; but in an intermediate 

 system of vessels, which, from their minute size (about 3^ of an inch in 

 diameter), are termed capillaries (capillus, a hair). The capillaries con- 

 stitute a microscopic network, which is distributed through every part" of 

 the body, so as to render it impossible to introduce the smallest needle 

 point beneath the skin without \vounding several of these fine vessels. It 

 is through the medium of the capillaries, that all the phenomena of nutri- 

 tion and secretion are performed. They are remarkable for their unifor- 

 mity of diameter, and for the constant divisions and communications 

 which take place between them, without any alteration of size. They 



* I have a preparation, showing the collateral circulation in a dog, in which I tied 

 the abdominal aorta ; the animal died from over-feeding nearly two years after the 

 operation 



