ARTERIES. 141 



they thus divide, the character of their walls gradually 

 undergoes such alterations, that at last minute canals, 

 the so-called capillary vessels, appear, provided with a 

 membrane as simple as any that is ever met with in the 

 body. The histological appearances which present them- 

 selves in these different vessels are as follows : 



On isolating an artery we find that its walls are rela- 

 tively very thick, and in those arteries which can be fol- 

 lowed with the naked eye, not only, the well-known 

 three coats are distinguished with the help of the micro- 

 scope, but in addition to these, a fine epithelial layer, 

 which invests the internal surface and is not wont to be 

 included in the class of structures usually termed coats. 

 The internal and external coats are essentially forma- 

 tions of connective tissue, which in the larger arteries 

 display a continually increasing quantity of elastic fibres ; 

 between them lies the relatively thick middle, or circu- 

 lar-fibre, coat, which, as being the seat of the muscular 

 fibres, constitutes what may be almost termed the most 

 important component of the arterial walls. These mus- 

 cular fibres are found in the greatest abundance in the 

 middle-sized and smaller arteries, whilst in the very 

 large ones, and especially in the aorta, elastic layers form 

 the predominant constituent even of the circular-fibre 

 coat. In small arteries, on microscopical examination, 

 there may be easily observed within this coat (Comp. 

 Figs. 26, b, b; 45, a) little transverse striations, corres- 

 ponding to the individual fibre-cells, arid encircling the' 

 vessel in such dense array that we find fibre -cell by the 

 side of fibre-cell without any interruption. The thick- 

 ness of this layer can be readily estimated in conse- 

 quence of the well-marked limits set to it upon the in- 

 and out-side by the longitudinal-fibre coats ; the only 

 deceptive appearance is presented by certain round 

 bodies, which are to be seen here and there in the sub- 



