262 STRUCTURE OF ARTERIES. 



by the surface of the entire 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 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 increase in frequency with the diminution in the 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 (ava between, rfrofjw mouth). This in- 

 crease in the frequency of anastomosis in the smaller branches is a 

 provision for counteracting the greater liability to impediment exist- 

 ing in them than in the larger branches. Where freedom of circu- 

 lation is of vital importance, this communication of the arteries is 

 very remarkable, as in the circle of Willis in the cranium, or in the 

 distribution of the arteries of the heart. It is also strikingly seen in 

 situations where obstruction is most likely to occur, as in the dis- 

 tribution to the alimentary canal, around joints, or in the hand and 

 foot. Upon this free communication existing every where 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 beautiful 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 inter- 

 mediate system of vessels, which, from their minute size, are termed 

 capillaries (capillus, a hair). The capillaries constitute a micro- 

 scopic 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 wounding several of these fine vessels. It 

 is through the medium of the capillaries that all the phenomena of 

 nutrition and secretion are performed. They are remarkable for 

 their uniformity of diameter, and for the constant divisions and 

 communications which take place between them without any altera- 

 tion of size. They inosculate on one hand with the terminal ram- 

 usculi of the arteries ; and on the other with the minute radicles of 

 the veins. 



Arteries are composed of three coats, external, middle, and in- 

 ternal The external or cellular coat is firm and strong, and serves 

 at the same time as the chief means of resistance of the vessel, and 

 of connexion to surrounding parts. It consists of condensed cellular 

 tissue, strengthened by an interlacement of glistening fibres which 



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

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

 operation. 



