ARTERIES. [ 



openings ; more rarely a very dense network 

 of principally longitudinal elastic fibres, with 

 narrow elongated fissures. 



The middle coat of the smaller arteries is 

 purely muscular. The fibres or fibre-cells, 

 which are connected into layers, may be 

 isolated by dissection, or by maceration and 

 boiling in a mixture of nitric acid with four 

 parts of water. 



The outer coat consists of areolar tissue 

 with elongated nuclei and fine elastic fibres, 

 and is nearly as thick as, or even thicker 

 than the middle coat. 



Magnified 350 Diameters. 



A small artery (a) and vein (6) (about 1-180" in diameter) from the me- 

 sentery of a child, after the addition of acetic acid : a, external 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 ; $, fibrous layer of elastic tissue . 



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 mid- 

 dle coat gradually loses its layers of muscular 

 fibres, until these and the fibres themselves 

 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 

 canbe distinguished consisting of their closely 

 aggregated nuclei. 



] ARTERIES. 



In moderate-sized arteries the middle coat 

 increases in thickness, but in addition to a 

 larger number of muscular layers, fine elastic 

 fibres in open networks are added, at first run- 

 ning somewhat irregularly through the muscu- 

 lar 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 fibres. The inner coat 

 sometimes contains between its elastic layer 

 and the epithelium several other layers, form- 

 ing with fine networks of elastic tissue more 

 externally situated in homogeneous granu- 

 lar 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 

 form of laminae. 



In the largest arteries, the 

 epithelial cells of the inner 

 coat are not so elongated, 

 and the inner coat consists 

 principally of layers of a ho- 

 mogeneous, striated, or even 

 distinctly fibrillar substance, 

 agreeing with areolar tissue, 

 traversed by finer and coarser 

 longitudinal networks of elas- 

 tic tissue. Immediately be- 

 neath the epithelium the net- 

 works of elastic fibres are 

 either very fine, or are re- 

 placed by one or more striated 

 layers, which when nucleated, 

 often appear as if composed 

 of fused epithelial cells, and 

 when homogeneous, resemble 

 pale elastic membranes. The 

 middle coat contains, as a 

 new element, elastic mem- 

 branes or plates, as many as 

 50 or 60, which, except in 

 their transverse direction, resemble the elas- 

 tic inner coat, sometimes forming the densest 

 networks of elastic fibres, at others fenes- 

 trated membranes. These layers alternate 

 with those of the muscular fibres traversed 

 by areolar tissue and networks of elastic 

 tissue. The muscular layer of the middle 

 coat is less developed, its cells smaller and 

 less regularly and perfectly formed. 



The outer coat is relatively and absolutely 

 thinner than that of the smaller; but the 

 structure is the same, except that its inner 

 elastic layer is much less developed. 



In some of the larger arteries of man, as 

 the axillary and popliteal, and the mesenteric 



