120 VASCULAR SYSTEM. 



dimensions, e. #., in large arteries of the mesentery, the intima 

 is seen in profile both in fresh preparations and after treatment 

 with bichromate of potash, as a doubly-contoured, sharply de- 

 fined membrane ; the surface view showing traces of longitu- 

 dinal fibres. In cross sections of smaller arteries, the intima 

 is seen as a wavy hyaline membrane, differing in thickness 

 according to the size of the artery. The intima of large veins 

 differs only in its thickness from that of the arteries. 



Muscular Coat. The muscular coat differs in its charac- 

 ters in different parts of the vascular system. In the arteries, 

 the muscular elements form layers which are connected to- 

 gether by the elastic lamellae interposed between them, the two 

 together constituting the tunica media. In many arteries 

 there are also bundles of muscular fibres in the intima, and in 

 others in the adventitia. In the minute arteries they form cir- 

 cular layers, the number of which varies according to the size 

 of the vessel. In such arteries the media is made up almost 

 entirely of muscular fibres. 



The muscular fibres of large arteries may be studied either 

 by teasing preparations of vessels steeped in bichromate of 

 potash, or in sections hardened in chromic acid. The muscle- 

 cells of large arteries appear, when isolated, to be broader, 

 relatively, than ordinary muscular elements, and are often split 

 at their ends into processes. The oblong nuclei are more or 

 less staff-shaped. If a portion of fresh bladder of the frog is 

 treated with acetic acid in the manner already recommended 

 in the chapter on unstripecl muscular fibres, or a portion of 

 mesentery of a frog or mammal with bichromate of potash or 

 acetic acid, the muscle-cells can be distinguished as trans- 

 versely arranged short spindles, inclosing long distinctly gran- 

 ular staff-shaped nuclei, which are arranged in rows alternating 

 with each other. In optical longitudinal sections of minute 

 arteries, such as occur very frequently in sections of hardened 

 tissues, the elements of the media exhibit the same appear- 

 ances as in cross sections of involuntary muscle in general ; as, 

 however, the muscle-cells are shorter, and their nuclei longer, 

 most cross sections exhibit a nucleus in almost every element. 

 In minute veins, muscular elements are seen which have a longi- 

 tudinal direction, but do not form a continuous laj'er. 1 



TSie intima and adventitia of the bloodvessels contain nu- 

 merous branched cells. To demonstrate them, sections must 

 be made of bloodvessels and treated with gold. They may bo 

 also shown in preparations made by the silver method. By this 

 method a rich network of lymphatics may be demonstrated in 

 the adventitia of the aorta of small animals. 



1 For the special arrangements of the muscular fibres in particular 

 arteries, the reader is referred to larger treatises on general anatomy. 



