GENERAL ANATOMY OP ARTERIES. 293 



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 

 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 (.vu. between, ffr'opa, mouth). This increase in the fre- 

 quency of anastomosis in the smaller branches is a provision for coun- 

 teracting the greater liability to impediment existing in them than in 

 the larger branches. Where freedom of circulation is of vital import- 

 ance, 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 distribution to the alimentary canal, 

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

 existing everywhere between arterial branches is founded the prin- 

 ciple 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 circu- 

 lation, in which, as is shewn 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 (about 

 a (/op of an inch in diameter), are termed capillaries (capillus, a hair). 

 The capillaries constitute 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 wound- 

 ing 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 alteration of size. They inosculate on 

 one hand with the terminal ramusculi of the arteries ; and on the 

 other with the minute radicles of the veins. 



Arteries are composed of three coats, external, middle, and internal. 

 The external or cellulo-fibrous coat is firm and strong, and serves at' 

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

 connection to surrounding parts. It consists of condensed fibro-cellu- 

 lar tissue, strengthened by an interlacement of glistening fibres which 



* I have a preparation, shewing 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. 



