CH. xviii.] THK CAPILLARIKS. 



The Capillaries. 



In all vascular textures except some parts of the corpora caver- 

 nosa of the penis, of the uterine placenta, and of the spleen, 

 the transmission of the blood from the minute branches of the 

 arteries to the minute veins is effected through a network of 

 capillaries. 



Their walls are composed of endothelium a single layer of 



Fig. 222. Surface view of an artery from the mesentery of a frog, ensheathed 

 in a peri-vascular lymphatic vessel, a, the artery, with its circular mus- 

 cular coat (media) indicated by broad transverse markings, with an indica- 

 tion of the advent itiii outside. /. lymphatic vessel ; its wall is a simple 

 endothelial membrane. Kl.-in and Noble Smith.) 



elongated flattened and nucleated cells, so joined and dovetailed 

 together as to form a continuous transparent membrane (fig. 223). 

 Here and there the endothelial cells do not fit quite accurately ; 

 the space is filled up with cement material ; these spots are 

 called pseudo-ttornata. 



The diameter of the capillary vessels varies somewhat in the 

 different tissues of the body, the most common size being about 

 :1 o',, ,, ? li f an m h ( i 2 JA). Among the smallest may be mentioned 

 those of the brain, and of the follicles of the mucous membrane 



