STRTJCTURE OF ARTERIES. 



169 



amount, or one may predominate, the other diminishing or even 

 disappearing altogether. 



Middle coat (fig. 107, c). 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 considerable 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 and in some small ones it is made up of many 

 layers; and elastic films either 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 than the rest, but the deeper tint is probably due to staining 

 by the blood after death. 



This coat consists mainly of muscular fibre cells (fig. 112 and fig. 113), 



Fig. 112. 



Fig. 113. 



Fig. 112. — Muscular Fibre-cells from HuM.iN Arteries. Magnified 350 Diameters. 



1. From the loopliteal artery ; a, natural ; b, treated with acetic acid. 2. From, a 

 small branch of the posterior tibial (from Kolliker). 



Fig. 113. — Muscular Fibre-cells from Superior Thyroid Artery (Man). 340 



Diameters. 



seldom more than from ^-i^ to ^^ of an inch long and fi'equcntly 

 presenting a very irregular shape with jagged extremities (fig. 113). 

 Their nuclei are markedly rod shaped and are often slightly curved. 

 Cells are occasionally met with, especially in the larger arteries, Avhich 

 appear to present transitions to the forked cells of which the muscular 

 substance of the heart is composed. Fine elastic fibres are also com- 

 monly to be found in this coat mixed with the muscular l)undles, and 

 traversing the layers in form of elastic networks, which in the larger 

 arteries pass into the elastic laminae already mentioned. 



The elastic fibres are accompanied by white fibres of areolar tissue in 



