THE 



SYSTEM. 



81 



fibres, and others present a more membranous character, marked by pale lines 

 having a longitudinal direction. In arteries of less than a line in diameter the 

 subepithelial layer consists of a single layer of stellate cells, and the connective 

 tissue is only largely developed in the large-sized vessels. The fenestrated mem- 

 brane in microscopic arteries is a very thin layer, but in the larger arteries, and 

 especially in the aorta, it has a very considerable thickness. 



The middle coat (tunica media) is distinguished from the inner by its color 

 and by the transverse arrangement of its fibres, in contradistinction to the longi- 

 tudinal direction of those of the inner coat. It consists of two varieties of struc- 

 ture. t/eUou' elastic tissue and mnxcular tissue, which are present in varying quan- 

 tities in different vessels, according to their size, the former tissue preponderating 

 in the larger vessels and the latter in the smaller ones. In the largest 

 arteries this coat is of great thickness, of a yellow color, and highly elastic; 

 it diminishes in thickness and becomes redder in color as the arteries become 

 smaller, and finally becomes very thin and disappears. In small arteries this 

 coat is purely muscular, consisting of muscle fibre-cells (Fig. 44) united to form 

 lamellae 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 one-tenth of a line 

 in diameter three or four layers. In arteries of medium size (Fig. 58) this coat 

 becomes thicker in proportion 



to the size of the vessel : its Y v <? 



layers of muscular tissue are 

 more numerous and inter- 

 mixed with numerous fine 

 elastic fibres which unite to 

 form broad-meshed networks. 

 In the larger vessels, as the 

 femoral, superior mesenteric. 

 coeliac axis, external iliac, 

 brachial and popliteal arte- 

 ries, the elastic fibres unite 

 to form lamellae, which alter- 

 nate with the layers of mus- 

 cular 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 pul- 

 monary artery, in which the 

 individual cells of the mus- 

 cular layer are imperfectly formed, while in the carotid, axillary, iliac, and sub- 

 clavian arteries the muscular layer of the middle coat is more developed. The 

 elastic lamellae are well marked, may amount to fifty or sixty in number, and 

 alternate regularly with the layers of muscular tissue. They are most distinct 

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

 and common carotid. In the larger arteries bundles of white connective-tissue 

 fibres have also been found in small quantity in the middle coat. 



The i'Xt> ///// coat (tunica adverttitta) 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 it is sometimes described as forming here, between the adventitia and 

 media, a special layer, the tunica elastica extern a 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 as thick or thicker than the middle coat. 



FIG. 58. An artery from the mesentery of a child, .062"', and b, 

 vein .067'" in diameter, treated with acetic acid and magnified 350 

 times, a. Tunica adventitia. with elongated nuclei, ft. Nuclei of 

 the contractile fibre-cells of the tunica media, seen partly from the 

 surface, partly apparent in transverse section, y. Nuclei" of the en- 

 dothelial cells. 6. Elastic longitudinal fibrous coat. 



