SMALLER BLOOD-VESSELS. 



113 



and the lining epithelium-cells, much elongated in both vessels, are far 

 longer and narrower in the small arteries than in the corresponding 

 veins (fig. 138). 



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 differences 





FlG. 137. A SMALL ARTERY, A, WITH A CORRESPONDING VEIN, B, TREATED WITH 



ACETIC ACID. (Kolliker.) (Magnified 350 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. 



between arteries and veins are still manifested. These differences may 

 be enumerated as follows : The veins are larger than the corresponding 

 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 com- 



H 



