52 CIRCULATION OF THE BLOOD 



disturbing influences or accidents. They have three well-defined coats. 

 As these vary considerably in arteries of different sizes, it will be con- 

 venient, in their description, to divide the vessels into three classes : 



1. The largest arteries; in which are included all that are larger 

 than the carotids and common iliacs. 



2. The arteries of medium size ; that is, between the carotids and 

 iliacs and the smallest. 



3. The smallest arteries; or those less than ^ to ^ of an inch (1.7 

 to 2. i millimeters) in diameter. 



The external coat of the arteries, called the tunica adventitia, is the 

 strongest of the three. It is composed of white fibrous tissue with a 

 network throughout its extent of yellow elastic fibres. A few longitudi- 

 nal muscular fibres are found in this coat. It is largely the external coat 

 that gives strength to the vessels. The nutrient vessels of the arteries 

 (vasa vasorum) ramify in the external coat. 



The middle coat is called the tunica media and is composed of both 

 muscular and elastic fibres. In the larger arteries it is the thickest coat 

 of the three. The muscular fibres are of the non-striated, or involuntary 

 variety and encircle the vessel. The general direction, also, of the 

 elastic fibres is transverse. These fibres interlace with each other in every 

 direction. Arteries of different sizes present differences in the thickness 

 and constitution of the middle coat. In the larger vessels, the elastic 

 elements predominate ; and in the smaller arteries, the muscular fibres 

 are more abundant. In the very smallest vessels of supply to the capil- 

 lary system, the middle coat presents muscular fibres only. The vaso- 

 motor nerves are distributed in the middle coat. 



The internal coat is called the tunica intima and is composed of three 

 layers. This is sometimes spoken of as the fenestrated membrane of 

 Henle. The innermost of the three layers of the intima is the endothe- 

 lial lining, made up of oblong endothelial cells with their long diameter 

 in the direction of the vessel. Just beneath the endothelial lining is the 

 subendothelial layer, composed of fine branching connective-tissue cor- 

 puscles and fibres. The external layer of the intima is a lamella of fine 

 fibres of elastic tissue. These layers are very thin in the smallest 

 arteries (see Plate III, Figs. 2, 3, 4). 



The elasticity of the arteries has an important influence on the general 

 circulation. It provides for what is practically a continuous flow of blood 

 from the smallest arteries into the capillary system. If it is possible to 

 imagine the arterial system as consisting of inert tubes, it is evident that 

 the intermittent force of the heart would be quite as apparent in the 

 small as in the large arteries ; but the elasticity of the walls of the ves- 

 sels, especially of the larger arteries, produces a recoil following the 



