182 THE BLOOD VASCULAE SYSTEM 



The tunica media of the arteriole forms two-thirds to three-fourths of 

 its wall, and consists almost entirely of firmly united smooth muscle 

 fibers. The adventitia, much thinner than the media, contains bundles 

 of white fibers and delicate interlacing elastic fibrils. 



Precapillary Arteries. The smallest arterioles pass into what may 

 be termed the precapillary arteries. In these minute vessels the wall 

 consists of scarcely more than the endothelial lining, about which is an 

 incomplete layer of circular muscle fibers, interspersed with occasional 



FIG. 201. A GROUP OP SMALL BLOOD-VESSELS. 



A, small artery obliquely cut; B, arteriole and venule, the latter filled with blood; 

 a, fat cells. A and B are from the connective tissue of the anterior cervical region. 

 Hematein and eosin. A, X HO; B, X 550. C, a small artery near the descending 

 aorta of man; the internal and external elastic membranes are rendered distinct by 

 the stain. Hematein, Weigert's elastic tissue stain, and picro-fuchsin. X 550. 



collagenous and elastic fibers. On approaching the capillaries the endo- 

 thelial tube is gradually laid bare. It is the smooth muscle which is 

 the last of the tissues to disappear from the arterial wall, whereas be- 

 yond the capillaries it is the fibrous tissues which are first added to 

 the endothelial tube to form the wall of the smallest venules (Fig. 207). 



Atypical Arteries. Certain atypical arteries differ markedly from 

 the typical structure above described. 



The umbilical arteries are almost exclusively muscular, and practically 

 lack elastic tissue. The muscle is arranged in two distinct layers : an inner 

 longitudinal, and a wide outer circular; external to these is usually a 



