clxx 



BLOOD-VESSELS. 



Middle coat. This consists of plain muscular tissue, in fine bundles, disposed 

 circularly round the vessel, and consequently tearing off in a circular direction, 

 although the individual Bundles do not form complete rings. The consider- 

 able thickness of the walls of the larger arteries is due chiefly to this coat ; 

 and in the smaller ones, it is said to be thicker in comparison with the calibre 

 of the vessel. In the largest vessels it is made up of many layers ; and 



shreds of elastic membrane, homogeneous, 

 finely reticular, or quite similar to the 

 fenestrated membrane of the inner coat, 

 are often found between the layers. The 

 middle coat is of a tawny or reddish-yellow 

 colour, not unlike that of the elastic tissue, 

 but, when quite fresh, it has a softer and 

 more translucent aspect. Its more internal 

 part is often described as redder thau the 

 rest, but the deeper tint is probably duo 

 to ctaiuing by the blood after death. This 

 coat is highly elastic, and was at one time 

 regarded by many, especially among the 

 French anatomists, as being identical in 

 nature with the yellow elastic tissue : but 

 it consists in reality of two constituents ; 

 namely, 1st, muscular fibre-cells, seldom 

 more than from -^^ to -^Q of an inch 

 long, collected in bundles, as already 

 stated ; 2ndly, fine elastic fibres mixed 

 with the muscular bundles and traversing 

 the muscular lasers in the form of elastic 

 net- works, which, in the larger arteries, pass 

 into the elastic laminae already mentioned, 

 The elastic fibres are accompanied by white 

 fibres of areolar tissue in small quan- 

 tity, the proportion of which increases with the size of the artery. It 

 is important further to note that the muscular tissue of the middle coat 

 is more pure in the smaller arteries, and that the admixture of other 

 tissues increases in the larger sized vessels ; in these, moreover, the 

 muscular cells are smaller. Accordingly, the vital contractility of the 

 arteries, which depends on their middle coat, is very little marked in 

 those of large size, but becomes much more conspicuous in the smaller 

 branches. 



External coat. This consists of two layers of different texture, viz. 

 1st, an internal stratum of genuine elastic tissue, most obvious in arte- 

 ries of medium calibre, and becoming thinner, and at length disappear- 

 ing in those of small size ; 2ndly, an outer layer, consisting of fine, 

 and closely felted bundles of white connective tissue, mixed with elastic 

 fibres. The bundles and fibres in large and middle-sized arteries chiefly 

 run diagonally or obliquely round the vessel, and their interlacement 

 becomes much more open and lax towards the surface of the artery, where 

 they connect the vessel with its sheath or with other surrounding parts, 

 This white fibrous layer is usually of great proportionate thickness in the 

 smaller arteries. 



Some arteries have much thinner coats than the rest, in proportion to 

 their calibre. This is strikingly the case with those contained within the 



Fig. XCII. MUSCULAR FIBRE CELLS 

 FROM HUMAN ARTERIES. MAGNI- 

 FIED 350 DIAMETERS. 



1. From the popliteal artery; a, 

 natural ; 6, treated with acetic acid. 

 2. From a small branch of the poste- 

 rior tibial (from Kolliker). 



