216 



THE CIRCULATORY SYSTEM 



[CH. XVIII. 



long axis of the artery ; while the elastic element, taking also a trans- 

 verse direction, is disposed in the form of closely interwoven and 

 branching fibres, which intersect in all parts the layers of muscular 

 fibres. In arteries of various sizes there is a difference in the pro- 

 portion of the muscular and elastic element, elastic tissue prepon- 

 derating in the largest arteries, and unstriped muscle in those of 

 medium and small size. 



(c) The internal coat is formed by a layer of elastic tissue, called 

 the fenestrated membrane of Henle. Its inner surface is lined with a 

 delicate layer of elongated endothelial cells (fig. 224, e), which make 

 it smooth, so that the blood may flow with the smallest possible 



amount of resistance from friction. Imme- 

 diately external to the endothelial lining 

 of the artery is fine connective tissue 

 (sub-endothelial layer) with branched cor- 

 puscles. Thus the internal coat consists 

 of three parts, (a) an endothelial lining, (b) 

 the sub-endothelial layer, and (c) elastic 

 layer. 



Vasa Vasorum. The walls of the 

 arteries are, like other parts of the body, 

 supplied with little arteries, ending in 

 capillaries and veins, which, branching 

 throughout the external coat, 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 sur- 

 rounded by a plexus of sympathetic nerves, 

 which twine around the vessel very much 

 They terminate in a plexus between the 



FIG. 225. Muscular fibre-cells 

 from human arteries, magni- 

 fied 350 diameters. (Kolliker.) 

 a. Nucleus, b. A fibre-cell 

 treated with acetic acid. 



like 



ivy 



round a tree. 



muscular fibres (fig. 227). 



The Veins. 



The venous system begins in small vessels which are slightly 

 larger than the capillaries from which they spring. These vessels 

 are gathered up into larger and larger trunks until they terminate 

 (as regards the systemic circulation) in the two venae cavae and the 

 coronary veins, which enter the right auricle, and (as regards the 

 pulmonary circulation) in four pulmonary veins, which enter the left 

 auricle. The total capacity of the veins diminishes as they approach 

 the heart ; but, as a rule, their capacity is two or three times that 

 of their corresponding arteries. The pulmonary veins, however, are 

 an exception to this rule, as they do not exceed in capacity the 

 pulmonary arteries. The veins are found after death more or less 



