296 



SPECIAL HISTOLOGY. 



003'"). In vessels of^'" in diameter, the fibre-cells, which are 

 united into lamella, may be pretty readily isolated by dissection, 

 and in still smaller ones by boiling and maceration in nitric 

 acid of 20 per cent., when they appear as delicate fibre cells 

 002— 003'" long, and 0002— 0-0025'" broad. The t. adven- 

 titia consists of connective tissue and fine elastic fibres, and is 

 usually as thick as the t. media or even a little thicker. 



The above description of the structure holds good of arteries 

 down to -'" in size, but nearer the capillaries the structure 

 changes more and more (fig. 280). Even in arteries ^'" in 

 diameter, the t. adventitia contains no elastic tissue, being com- 

 posed only of connective tissue with elongated nuclei, which at 

 first still retains its fibrous character, but afterwards, though 

 always nucleated, appears more homogeneous, and ultimately 

 represents a thin, truly structureless membrane, which, in vessels 

 under 0*007''' disappears altogether. The annular fibrous coal 



in arteries less than ~" t down to 

 those having a diameter of ^'", 

 still presents 2 — 3 layers of muscles 

 and a thickness of 0*005— 0'008'"; 

 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"', appear only in the form 

 of short, elongated or elliptical cells 

 of 0*015 — 0*006"', with shorter nuclei. 

 1 1 'Mi I2B SSI JIT "*" n vesse ^ s °f 0*01 2'" in diameter, these 

 / SPy^iSP^M more embryonic forms of contractile 



fibre-cells still constitute a connected 

 lamina, but after that, they are gra- 

 dually separated from each other (fig. 280) and become wholly 

 lost. The /. intima, in vessels more than 0*028 — 0'03'" in size, 

 has an elastic inner membrane, which, however, in its earliest 

 form, is very delicate, and does not appear to attain its full deve- 

 lopment in arteries less than 0*06 — 0*08"' in diameter. On the 

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



Fig. 280. An artery, a, of 001'", and a vein, b, of 0-015'", from the mesentery of 

 a child, x 350 diam., treated with acetic acid. The letters as in fig. 279 : e, t. media 

 of the vein, composed of nucleated connective tissue. 



