STRUCTURE OF ARTERIES 



499 



I iliac, femoral, and carotid, elastic fibers unite to form lamellae which alternate with the layers 

 of muscular fibers; these lamellae are united to one another by elastic fibers which pass between 

 the muscular bundles, and are connected with the fenestrated membrane of the inner coat (Fig. 

 450). In the largest arteries, as the aorta and innominate, the amount of elastic tissue is very 

 considerable; in these vessels a few bundles of white connective tissue also have been found in 

 the middle coat. The muscle fiber cells are about oO.u 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 fibers in all but the smallest arteries. The elastic 

 tissue is much more abundant next the tunica media, 

 and it 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 of 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 connec- 

 tive tissue and elastic fibers; while in the smallest 

 arteries, just above the capillaries, the elastic fibers 

 are wanting, and the connective tissue of which the 

 coat is composed becomes more nearly homogeneous 

 the nearer it approaches the capillaries, and is grad- 

 ually reduced to a thin membranous envelope, which 

 finally disappears. 



Some arteries have extremely thin walls in propor- 

 tion to their size; this is especially the case in those 

 situated in the cavity of the cranium and vertebral 

 canal, the difference depending on the thinness of the 

 external and middle coats. 



The arteries, in their distribution throughout the 

 body, are included in thin fibro-areolar investments, 

 which form their sheaths. The vessel is loosely con- 

 nected with its sheath by 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, like the other organs of the body, are supplied with bloodvessels. These 

 nutrient vessels, called the vasa vasorum, arise from a branch of the artery, or from a neighbor- 

 ing 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; in some of the larger mammals 

 a few vessels have been traced into the middle coat. Minute veins return the blood from these 

 vessels; they empty themselves into the vein or veins accompanying the artery. Lymphatic 

 vessels are also present in the outer coat. 



Arteries are also supplied with nerves, which are derived from the sympathetic, but may pass 

 through the cerebrospinal nerves. 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 distributed principally 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 cavernous structure 

 of the sexual organs, of the splenic pulp, and of the placenta) terminate in net-works of vessels 

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

 capillaries. They are interposed between the smallest branches of the arteries and the commenc- 

 ing veins, constituting a net-work, the branches of which maintain the same diameter throughout; 

 the meshes of the net-work are more uniform in shape and size than those formed by the anasto- 

 moses of the small arteries and veins. 



The diameters of the capillaries vary in the different tissues of the body, the usual size being 

 about 8,u. The smallest are those of the brain and the mucous membrane of the intestines; 

 and the largest those of the skin and the marrow of bone, where they are stated to be as large 

 as 2(V in diameter. The/or/?i of the capillary net varies in the different tissues, the meshes being 

 generally rounded or elongated. 



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 are not of an 

 absolutely circular outline, but more or less angular, sometimes nearly quadrangular, or polygonal, 

 or more often irregular. 



Fio. 449. Small artery and vein, pia mater of 

 sheep. X 250. Surface view above the inter- 

 rupted line; longitudinal section below. Artery 

 in red; vein in blue, 



