SMALLER BLOOD-VESSELS 



93 



and narrower in the small arteries than in the corresponding veins 

 (fig. 117). 



In the smallest vessels it will be found that the elastic layer has dis- 

 appeared in the veins, and the muscular tissue is considerably reduced 

 in thickness in both kinds of vessels. Indeed, it is soon represented 

 by but a single layer of contractile cells, and even these no longer 

 form a complete layer. By this time also, the outer coat and the 

 elastic layer of the inner coat have entirely disappeared both from 

 arteries and veins. The vessels are reduced, therefore, to the condition 

 of a tube formed of pavement epithelium cells, with a partial covering 

 of circularly disposed muscular cells. 



Even in the smallest vessels, which are not capillaries, the differ- 



c A b 



FIG. 116. A SMALL ARTERY, A, WITH A CORRESPONDING VEIN, B, TREATED WITH 



ACETIC ACID. (Magnified o50 diameters.) 



a, external coat with elongated nuclei ; 6, nuclei of the transverse muscular tissue of the 

 middle coat (when seen endwise, as at the sides of the vessel, their outline is circular) ; 

 c, nuclei of the epithelium-cells ; d, elastic layers of the inner coat. 



ences between arteries and veins are still manifested. These differences 

 may be enumerated as follows : The veins are larger than the corre- 

 sponding arteries ; they branch at less acute angles ; their muscular 

 cells are fewer, and their epithelium-cells less elongated ; the elastic 

 layer of the inner coat is always less marked, and sooner disappears. 



Capillary vessels. When traced to their smallest branches, the 

 arteries and veins eventually are seen to be continued into a network 

 of the smallest blood-vessels or capillaries. The walls of these are 

 composed only of flattened epithelium-cells (fig. 118) continuous with 

 those that line the arteries and veins ; these cells can be exhibited by 

 staining a tissue with nitrate of silver. The capillaries vary somewhat 

 in size and in the closeness of their meshes ; their arrangement in 



