THE BLOODVESSELS., 



501 



areolar tissue, with an elastic inuer layer. In the (2<%) medium- 

 sized arteries (f of a line to 2 or 3 lines) this coat is thicker than 

 the middle coat, being %fo to ^ of an inch thick. It has an inner 

 portion, in the form of a laminated elastic membrane, in the largest 

 arteries of this class (brachial, femoral, &c.). The external portions 

 of the external coat also abound in elastic fibres, sometimes pre- 

 senting a laminated aspect. 3c%. In the small arteries (less than 

 of a line in diameter), the external tunic is merely a layer of areolar 

 tissue, as thick or thicker than the tunica media. In arteries T \ of 

 a line or less in diameter, however, the outer coat contains no elas- 

 tic fibres, but only collagenous tissue and elongated nuclei; and 

 which, though still nucleated, at length, towards the capillaries, 

 become homogeneous, then a thin simple membrane, and finally, 

 in vessels under of an inch, disappear altogether. 



Fig. 333. 



Section of the aorta of the ox, showing the arrangement of the two layers of the longitudinal 

 fibrous tunic, and of the circular fibrous tunic, a and 6, the inner coat : a, the epithelial layer ; 

 b, the internal portion of the longitudinal fibrous layer (fenestrated membrane, Henle). c and d, 

 the middle coat ; c, the external coarse stratum of the longitudinal fibrous layer ; d, a small portion 

 of the circular fibrous tunic ; most of the fibres are cut across, but a few which take an oblique 

 course, are seen in their whole length, an.d their peuniform branching is slightly indicated. (Longi- 

 tudinal section.) 



2. The middle coat of large arteries (Fig. 833) consists of plates of 

 elastic tissue, of collagenous tissue, fine elastic networks, and smooth 

 muscular fibres; the last constituting only one-fourth to one-third 



