1128 GENERAL ANATOMY OR HISTOLOGY. 



tissue is very considerable. In these vessels also bundles of white connective 

 tissue have been found in small quantities in the middle coat. The muscle-fibre 

 cells of which the middle coat is made up are about ^^ of an inch in length, and 

 contain well-marked, rod-shaped nuclei, which are often slightly curved. 



The external coat (tunica adventitia) consists mainly of fine and closely felted 

 bundles of white connective tissue, but also contains elastic fibres in all but the 

 smallest arteries. The elastic tissue is much more abundant next the tunica media, 

 and is sometimes described as forming here, between the adventitia and media, a 

 special layer, the tunica elastica externa of Henle. This layer is most marked in 

 arteries oi medium size. In the largest vessels the external coat is relatively 

 thin ; but in small arteries it is of greater proportionate thickness. In the smaller 

 arteries it consists of a single layer of white connective tissue and elastic fibres; 

 while in the smallest arteries, just above the capillaries, the elastic fibres are want- 

 ing, and the connective tissue, of which the coat is composed, becomes more homo- 

 geneous the nearer it approaches the capillaries, and is gradually reduced to a thin 

 membranous envelope which finally disappears. 



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 on the greater thinness of the external and 

 middle coats. 



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

 fibro-areolar 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 downward of the transversalis and iliac 

 fasciae 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 encloses 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 blood-vessels like the other organs of 

 the body ; they are called the 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 

 coat, but do not, in man, penetrate the other coats ; though in some of the larger 

 mammals some few vessels have been traced into the middle coat. Minute veins 

 serve to return the blood from these vessels ; they empty themselves into the vein 

 or veins accompanying the artery. Lymphatic vessels and lymphatic spaces are 

 also present in the outer coat. 



Arteries are also supplied with nerves, which are derived chiefly from the sym- 

 pathetic, but partly from the cerebro-spinal system. They form intricate plexuses 

 upon the surfaces of the larger trunks, and run along the smaller arteries as single 

 filaments or bundles of filaments, which twist around the vessel and unite with 

 each other in a plexiform manner. The branches derived from these plexuses 

 penetrate the external coat, and are principally distributed to the muscular tissue 

 of the middle coat, and thus regulate, by causing the contraction and relaxation 

 of this tissue, the amount of blood sent to any part. 



The Capillaries. The smaller arterial branches (excepting those of the cavern- 

 ous structure of the sexual organs, of the spleen, 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 com- 

 mencing veins, constituting a network, 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 ^Vo" of an inch. The smallest are those of the brain and 



