4 88 



STRUCTURE OF THE 



[sect. 214. 



0-03'" to 0-007'", they lapse into short, elongated, or oblong cells, 

 0-015'" to o*oo6'", whose nuclei have no longer their elongated 

 form. These more embryonic forms of contractile fibre-cells form 

 a connected layer, as far as to vessels of o - oi2'", but then gradually 

 separate from one another (fig. 198), and completely disappear. — 

 The tunica intima possesses an elastic coat on vessels as small as 

 0-028'" to 003'" in diameter ; it is, of course, very delicate on its 

 first appearance, and appears in a fully developed state only on 

 arteries of o'o6" to 0*08'". The epithelium, however, can be 

 traced on minute arteries of 0*07'", or even o'oi'" in diameter; 

 but, it is to be observed, that its cells become at last incapable of 

 isolation, so that their separate existence is only to be inferred 

 from the presence of longitudinally oval nuclei placed closely 

 together. 



Arteries of medium size, above four-fifths of a line, or a line, up 



to those of two or three lines in diameter, present at first but little 



change in the outer and inner coats, but a considerable difference 



Fig. 199. is found in the tunica media, 



which not only becomes thicker 

 with the increase in size of the 

 vessels (reaching 0'03'" to 0'i2"'), 

 but is also altered in structure. 

 Its muscular layers become in- 

 creasingly numerous, though their 

 elements . remain identical with 

 those described in the smaller 

 vessels ; but along with the mus- 

 cular fibres fine elastic fibres ap- 

 pear in the tunica media, and 

 these unite to form wide-meshed networks, which, at first, run 

 separately and without order among the muscular elements ; but 

 in the larger of the arteries at present under review, these elastic 

 elements are accompanied by some connective tissue, and occasion- 

 ally exhibit an inclination to form special laminae, alternating with 

 the muscular layers, without, however, losing the character of a 

 continuous network pervading the whole middle coat. Thus the 

 tunica media already loses its exclusively contractile structure, 

 though it is to be admitted that the muscular fibres still retain a 

 considerable preponderance. — The tunica intima of the medium- 

 sized arteries has not unfrequently several layers between the 

 elastic inner coat and the epithelium, among which the above- 

 described striped lamellae are the most conspicuous. These lamellae, 



Transverse section of the human ait.pru- 

 funda femoris ; magnified 30 times, a. Tu- 

 nica intima, with the elastic layer, (the 

 epithelium is not visible); 6. tunica media, 

 without elastic lamellae, but with fine plastic 

 fibres ; c. tunica adventitia, with elastic net- 

 works and connective tissue. 



