AETERIES 



89 



elastic membrane is represented only by a very incomplete layer 

 of elastic fibres. The tunica media of the arteriole forms two- 

 thirds to three-fourths of its wall, and consists almost entirely of 

 firmly united smooth muscle fibres. The adventitia, much thinner 

 than the media, contains bundles of white fibres and delicate inter- 

 lacing elastic fibrils. 



The smallest arfcerioles 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 in- 

 complete layer of circular muscle fibres, interspersed with occa- 

 sional white fibrous and elastic fibres. On approaching the capil- 

 laries the endothelial tube is gradually laid bare. It is the smooth 



A 



A GROUP OF SMALL BLOOD VESSELS. 



.4, small artery obliquely cut; B, arteriole and venule, the latter filled with blood; o, 

 fat cells. A and B are from the connective tissue of the anterior cervical region. Hema- 

 tein and eosin. A, x 110; 5, x 550. (7, a small arteriole 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. 



muscle which is the last of the tissues to disappear from the arte- 

 rial wall, whereas beyond the capillaries it is the fibrous tissues 

 which are first added to the endothelial tube to form the wall of 

 the smallest venules (Fig. 94). 



Comparison of Large and Small Arteries. The larger arteries 

 are typically elastic, the smaller typically muscular. In the larger 

 vessels the elastic tissue forms about one-half of the entire wall ; 

 toward the smaller arteries this tissue progressively diminishes 

 until, in the arterioles, it is limited to an incomplete internal 



