HAT. X 



vkK or iii.oon-YKssKi.s 



95 



dctinite layer, a muscular coat, as it is called, round the endo- 

 thclium v and there is, in addition, a little, fine connective tissue 

 outside this. In arteries still larger the muscular coat becomes 

 still thicker and consists of several layers, between which are 

 layers composed of networks of elastic fibres, and there is a 

 special layer of this elastic material immediately outside theendo- 

 thclium. Outside the muscular coat the connective tissue layer 

 is also thicker. So that the wall of an artery consists of three 

 layers: (i) the endothclium on the inner side, with a thin 

 elastic layer on the outer side of it ; (2) the muscular and 

 elastic coat ; (3) the connective tissue coat on the outer side. 



' OO 



B * 



Fi ".. 46. Capillaries. 



A, surface view ; H, cut lengthwise ; C, cut across ; e.c, endothelial cells ; n, nuclei ; 

 i/, the lumen or bore. 



The large arteries, especially the aorta, contain, as compared 

 with smaller arteries, much elastic tissue in proportion to 

 muscular tissue. Owing to the presence of this elastic tissue, 

 all the arteries are elastic ; they yield when pulled, and go 

 back to their former condition when let go. When fluid is 

 driven into them they distend, and when the fluid is let out, 

 shrink aj^ain. 



Veins. The walls of veins are in structure similar to 

 the walls of arteries, and consist as a rule of the same three 

 layers. The walls are not so thick as those of arteries, for 

 there is much less elastic tissue and less muscular tissue than 

 there is in arteries, though there is more connective tissue. 

 When a vein is cut across the vessel collapses, that is, the thin 



