THE ARTERIES. 73 



Ham, but increase in thickness from within outwards. The internal and middle 

 coats are separated, by either a dense elastic reticulated coat, or a true fenes- 

 trated membrane. 



The middle coat is distinguished from the inner by its color, and by the trans- 

 verse arrangement of its fibres, in contradistinction to the longitudinal direction 

 of those of the inner coat. In the largest arteries, this coat is of great thick- 

 ness, of a yellow color, and highly elastic; it diminishes in thickness, and 

 becomes redder in color as the arteries become smaller; becomes very thin, and 

 finally disappears. In small arteries, this coat is purely muscular, consisting 

 of muscular fibre-cells united to form lamella? which vary in number according 

 to the size of the artery, the very small arteries having only a single layer, and 

 those not larger than the -^th of a line in diameter, three or four layers. In 

 arteries of medium size, this coat becomes thicker in proportion to the size of 

 the vessel ; its layers of muscular tissue are more numerous, and intermixed 

 with numerous fine elastic fibres which unite to form broad-meshed networks. 

 In the larger vessels, as the femoral, superior mesenteric, coeliac, external iliac, 

 brachial, and popliteal arteries, the elastic fibres unite to form lamellae, which 

 alternate with the layers of muscular fibre. In the largest arteries, the muscular 

 tissue is only slightly developed, and forms about one-third or one-fourth of the 

 whole substance of the middle coat ; this is especially the case in the aorta, and 

 trunk of the pulmonary artery, in which the individual cells of the muscular 

 layer are imperfectly formed ; while, in the carotid, axillary, iliac, and subclavian 

 arteries, the muscular tissue of the middle coat is more developed. The elastic 

 lamella? are well marked, may amount to fifty or sixty in number, and alternate 

 regularly with the layers of muscular fibre. They are most distinct, and 

 arranged with most regularity in the abdominal aorta, innominate artery, and 

 common carotid. 



The external coat consists mainly of connective tissue, and contains elastic 

 fibres in all but the smallest arteries. In the largest vessels, the external coat 

 is relatively thin; but in small arteries, it is as thick, or thicker, than the mid- 

 dle coat. In arteries of the medium size, arid above it, the external coat is 

 formed of two layers, the outer of which consists of connective tissue, con- 

 taining an irregular elastic network, while the inner is composed of elastic tissue 

 onlv- 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?. In the smaller arteries, the former layer of mixed connective tis- 

 sue and elastic fibres composes the whole of the external tunic ; while in the 

 smallest arteries just above the capillaries, the elastic fibres are wanting, and 

 the connective tissue of which the coat is composed becomes more homoge- 

 neous 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 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 trans versa] is and 

 iliac fascia? 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 dis- 



