THE CIKCULATION OF THE BLOOD. 109 



Immediately external to the endothelial lining of the artery is fine 

 connective tissue, sub-endothelial layer, with branched corpuscles. Thus 

 the internal coat consists of three parts (a) an endothelial lining, (#) the 

 sub-en dothelial layer, and (c) elastic layers. 



Vasa Vasorum. The walls of the arteries, with the possible excep- 

 tion of the endothelial lining and the layers of the internal coat imme- 

 diately outside it, are not nourished by the blood which they convey, but 

 are, like other parts of the body, supplied with little arteries, ending in 

 capillaries and veins, which, branching throughout the external coat, 



FIG. 103. Ramification of nerves and termination in the muscular coat of a small artery of the 

 frog (Arnold). 



extend for some distance into the middle, but do not reach the internal 

 coat. These nutrient vessels are called vasa vasorum. 



Nerves. Most of the arteries are surrounded by a plexus of sympa- 

 thetic nerves, which twine around the vessel very much like ivy round 

 a tree: and gangli are found at frequent intervals. The smallest arte- 

 ries and capillaries are also surrounded by a very delicate network of 

 similar nerve-fibres, many of which appear to end in the nuclei of the 

 transverse muscular fibres (Fig. 103). 



III. The Capillaries. 



Distribution. In all vascular textures except some parts of the cor- 

 pora cavernosa of the penis, and of the uterine placenta, and of the 

 spleen, the transmission of the blood from the minute branches of the 

 arteries to the minute veins is effected through a network of capillaries. 

 They may be seen in all minutely injected preparations. 



The point at which the arteries terminate and the minute veins com- 



