ARTERIES. 



[ 74 ] 



ARTERIES. 



small elongated openings ; more rarely a 

 very dense network of principally longitudi- 



34. 



Fig. 



Magnified 3.50 diami ters. 

 ^A small artery (a) and Tein (J) (about 1-lSO" in diameter) from the me- 

 sentery of a child, after the addition of acetic acid : a, exfernal coat, with 

 elongated nuclei ; /3, nuclei of the muscular fibres of the middle coat, partly 

 seen from the surface, partly the sectional view ; y, nuclei of the epithelial 

 cells ; S, fibrous layer of elastic tissue. 



nal elastic fibres, with narrow 

 fissures. 



Flo-. .35. 



elongated 



Magnified 350 diameters. 

 Muscular fibre-cells from human arteries. 1, from 

 the popliteal ; a, before, b, after the addition of acetic 

 acid; 2, from a twig of the anterior tibial artery: a, 

 nuclei. 



The middle coat of the smaller arteries is 

 purely muscular. The fibres or fibre-cells, 

 W'hich are transverse and 

 connected into layers, may 

 be isolated by dissection, 

 or by maceration and boil- 

 ing in a mixture of nitric 

 acid with four parts of 

 water. 



The outer coat consists 

 of connective tissue with 

 elongated nuclei and fine 

 elastic fibres, and is nearly 

 as thick as, or even thicker 

 than the middle coat. 



In the smallest arteries, 

 the outer coat gradually 

 ceases to contain elastic 

 tissue, consisting merely of 

 areolar tissue and the nu- 

 clei ; this gradually loses 

 its fibrous character, next 

 becoming homogeneous, 

 and finally a thin perfectly 

 structureless membrane, 

 and disappearing. In the 

 same manner the middle 

 coat gradually loses its lay- 

 ers of muscular fibres, until 

 these and the fibres them- 

 selves ultimately vanish. 

 On tracing the smaller 

 arteries downwards, the inner coat is first 

 found to lose its' elastic fibres ; and at last 

 the epithelial cells cease to be isolable, all 

 that can be distinguished consisting of their 

 closely aggregated nuclei ; but by macera- 

 tion in very dilute solution of nitrate of sil- 

 ver, the lines of demarcation of the cell- 

 walls are rendered beautifully distinct. 



In 7no(h'ratc-sized avtevies the middle coat 

 increases in thickness, but, in addition to a 

 larger number of muscular layers, fine elas- 

 tic fibres in open networks are added, at 

 first running somewhat irregularly through 

 the muscular elements, and in the larger 

 vessels of this category mixed with areolar 

 tissue and here and there forming layers 

 alternating with those of the muscular fi- 

 bres. The inner coat sometimes contains 

 between its elastic layer and the epithelium 

 several other layers, forming, with fine net- 

 works of elastic tissue more externally 

 situated in homogeneous granular or fibrillar 

 areolar tissue, a strong middle layer, the 

 elements of. which are longitudinal. The 

 outer coat in these vessels contains more 

 elastic tissue, in the fqi-m of laminte. 



