680 



SPECIAL HISTOLOGY. 



The above description of the structure holds good of arteries down to 

 1-8 of a line in size, but nearer the capillaries the structure changes 

 more and more (Fig. 280). Even in arteries 1-10 of a line in diameter, 

 the t. advent it ia contains no elastic tissue, being composed only of con- 

 nective tissue with elongated nuclei, which at first still retains its fibrous 

 character, but afterwards, though always nucleated, appears more homo- 

 geneous, and ultimately represents a thin, truly structureless membrane, 

 which, in vessels under 0-007 of a line disappears altogether. The annu- 

 lar for OILS coat in arteries less than 1-10 of a 

 line, down to those having a diameter of 1-25 

 of a line, still presents 23 layers of muscles 

 and a thickness of 0-005-0-008 of a line; 

 in smaller vessels it has but one layer, the 

 elements of which at the same time become 

 shorter and shorter; and finally, in vessels 

 between 0-03-0-007 of a line, appear only in 

 the form of short, elongated or elliptical cells 

 of 0-015-0-006 of a line with shorter nuclei. 

 In vessels of 0'012 of a line in diameter, 

 these more embryonic forms of contractile 

 fibre-cells still constitute a connected lamina, 

 but after that, they are gradually separated 

 from each other (Fig. 280) and become wholly 

 lost. The t. intima, in vessels more than 



0-028-0-03 of a line in size, has an elastic inner membrane, which, how- 

 ever, in its earliest form, is very delicate, and does not appear to attain 

 its full development in arteries less than 0-06-0-08 of a line in diameter. 

 On the other hand, the epithelium may be traced in vessels of not more 

 than 0-01 or even of 0-07 of a line ; and it may at the same time be 

 remarked, that its cells can no longer be isolated, and its presence only 

 be recognized from the closely placed elliptical nuclei. 



Medium-sized arteries above 4-5 or 1 line, up to those of 2 and 3 lines 

 in diameter, at first, present no great alterations in the external and in- 

 ternal tunics, whilst the t. media is not^onlj: always increased in thick- 

 ness in proportion to the size of the vessel (from 0-05-0-12 of a line), but 

 is also changed in structure. For now, in addition to muscular layers, the 

 number of which constantly increases, but the elements of which are pre- 

 cisely the same as before, we observe fine elastic fibres, which, united into 

 wide-meshed networks, run, at first singly and with little regularity 

 among the muscular elements ; but in larger vessels of this -category 



FIG. 280. An artery, a, of 0-01, and a vein, 6, of 0-015 of a line, from the mesentery of 

 a child, magnified 350 diameters, treated with acetic acid. The letters as in Fig. 279: i, t. 

 media of the vein, composed of nucleated connective tissue. 



