EPITHELIAL AND ENDOTHELIAL TISSUE. 337 



According to C. Toldt, the muscle-fibers of the middle coat 

 are wanting in the initial portion of the aorta, in the pulmonary 

 artery, and in the small arterioles of the retina ; in the aorta 

 descendens, the A. iliaca communis, and the A. poplitea small 

 muscle bundles, in an oblique or a longitudinal direction, are 

 interspersed between the circular ones j and in other arteries (A. 

 renalis, lienalis, spermatica interna), at the inner boundary of 

 the muscle coat, scanty longitudinal bundles occur, which, by 

 some authors, are considered to belong to the inner coat. Some- 

 times, in the corresponding arteries of different persons, slight 

 differences are observed in the distribution of the muscles of the 

 middle coat. 



The outer coat is composed of fibrous connective tissue, which 

 varies greatly, both in amount and density, in different vessels. 

 In the smallest arterioles it is only slightly developed, while in 

 the largest arteries it attains considerable breadth. In medium- 

 sized arteries, the muscle layer toward the connective tissue is 

 bounded by a hyaline, elastic membrane, which in the large 

 arteries is widened into a thick layer of dense fibrous connective 

 tissue. The bundles of this tissue are interlacing, and at their 

 boundaries marked by a coarse, well-developed reticulum, or 

 frame-work of elastic substance. This frame is denser in the 

 neighborhood of the middle coat than toward the peripheral 

 portion. The latter exhibits scattered bundles of smooth muscles 

 in a longitudinal arrangement, and is gradually transformed into 

 loose connective tissue, which composes the sheath of the artery, 

 and carries a system of capillary blood-vessels, the so-called vasa 

 vasorum. 



The development of arteries was studied by J. B. Greene, who 

 found that they are originally solid cords, composed of indifferent 

 or embryonal elements. The most central ones become vacu- 

 oled ; others are transformed into a flat layer to form the future 

 endothelia ; while the most external are elongated in shape, and 

 assume a circular arrangement, this being the first trace of the 

 future muscle-fibers. (See Art. on menstrual decidua.) 



(3) The Coats of Veins are similar in composition to those of 

 arteries ; but the elastic substance and the muscle tissue is less 

 developed in veins than in arteries. It is the middle coat, espe- 

 cially, which in veins varies most in breadth. This coat is also 

 comparatively more developed in the veins of the lower than in 

 those of the upper extremities. In the V. portae, cava asceudens, 

 and hepatica the muscle coat is only faintly defined, and its 

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