360 ARTERIES. 



connective tissue and elastic fibres. It is very thin in the largest arteries ; but in 

 those of medium size, and in small arteries, is as thick as, or thicker than the middle 

 coat. In small arteries, this coat consists of connective tissue and fine elastic fibres. 

 In arteries rather larger than the capillaries, the elastic fibres are wanting ; the 

 connective tissue composing the coat becoming, the nearer it approaches the capil- 

 laries, more homogeneous, being gradually reduced to a thin membranous envelope 

 which finally disappears. In articles of medium size, this coat is composed of 

 two distinct layers, an inner composed of elastic tissue ; an outer, composed of 

 connective tissue, containing elastic networks irregularly connected with each 

 other. The inner elastic layer is very distinct in the carotid, femoral, brachial, 

 profunda, mesenteric, and cceliac arteries, the elastic fibres being often arranged 

 in lamella?. . . 



Some arteries have extremely thin coats in proportion to their size ; this is 

 especially the case in those situated in the cavity of the cranium and spinal canal, 

 the difference depending upon the greater thinness of the external and middle 

 coats. 



The arteries, in their distribution throughout the body, are included in a thin 

 areolo-fibrous investment, which forms what is called their sheath. In the limbs, 

 this is usually formed by a prolongation of the deep fascia ; in the upper part of 

 the thigh, it consists of a continuation downwards of the transversalis and iliac 

 fasciaj of the abdomen ; in the neck, of a prolongation of the deep cervical fascia. 

 The included vessel is loosely connected with its sheath by a delicate areolar 

 tissue, and the sheath usually incloses the accompanying veins, and sometimes a 

 nerve. Some arteries, as those in the cranium, are not included in sheaths. 



All the larger arteries are supplied with bloodvessels like the other organs of 

 the body ; they are called vasa vasorum. These nutrient vessels arise from a 

 branch of the artery or from a neighboring vessel, at some considerable distance 

 from the point at which they are distributed ; they ramify in the loose areolar 

 tissue connecting the artery with its sheath, and are distributed to the external 

 and middle coats, and, according to Arnold and others, supply the internal coat. 

 Minute veins serve to return the blood from these vessels ; they empty themselves 

 into the venae comites in connection with the artery. Arteries are also provided 

 with nerves ; they are derived chiefly from the sympathetic, but partly from the 

 cerebro-spinal system. They form intricate plexuses upon the surface of the larger 

 trunks, the smaller branches being usually accompanied by single filaments ; their 

 exact mode of distribution is unknown. According to Kolliker, the majority of 

 the arteries of the brain and spinal cord, those of the choroid, of the placenta, as well 

 as many arteries of muscles, glands, and membranes, are unprovided with them. 



The smaller arterial branches, excepting those of the cavernous structures of 

 the sexual organs, and in the uterine placenta, terminate in a network of vessels 

 which pervade nearly every tissue of the body. These vessels, from their minute 

 size, are termed capillaries (capillus, " a hair"). They are interposed between the 

 smallest branches of the arteries and the commencing veins, constituting a net- 

 work, the branches of which maintain the same diameter throughout, the meshes 

 of the network being more uniform in shape and size than those formed by the 

 anastomoses of the small arteries and veins. 



The diameter of the capillaries varies in the different tissues of the body, their 

 usual size being about ^V^th part of an inch. The smallest are those of the 

 brain, and the mucous membrane of the intestines ; the largest, those of the skin, 

 and the marrow of bones. 



The form of the capillary net varies in the different tissues, being modifications 

 chiefly of rounded or elongated meshes. The rounded form of mesh is most 

 common, and prevails where there is a dense network, as in the lungs, in most 

 glands and mucous membranes, and in the cutis ; the meshes being more or less 

 angular, sometimes nearly quadrangular, or polygonal ; more frequently, irregular. 

 Elongated meshes are observed in the bundles of fibres and tubes composing 

 muscles and nerves, the meshes being usually of a parallelogram form, the long 



