222 THE CIRCULATORY SYSTEM [CH. XIX. 



in the arteries of the omentum, mesentery, membranes of the brain, 

 lung, liver and spleen, the spaces are continuous with vessels which 

 distinctly ensheath them perivascular lymphatics (fig. 203). 



The Capillaries. 



In most cases the blood finds its way from the small arteries to 

 the small veins through a network of minute cylindrical vessels 

 called capillaries. But in certain cases (parathyroid, spleen, the 

 thyroid of some animals, erectile tissue, the placenta, and the 

 embryonic liver and kidney) the connecting system of vessels are 

 larger and have an irregular shape ; these vessels are termed sinusoids. 



The walls of both capillaries and sinusoids are composed of 

 endothelium a single layer of elongated flattened and nucleated 

 cells, so joined and dovetailed together as to form a continuous 



FIG. 204. Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothelial 

 membrane of which they are composed. (Klein and Noble Smith.) 



transparent membrane (fig. 204). Here and there the endothelial 

 cells do not fit quite accurately; the space is filled up with cement 

 material ; these spots are called pseudo-stomata. 



The diameter of the capillary vessels varies somewhat in the 

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

 -ooVu-th f an i ncn (12 /x). Among the smallest may be mentioned 

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

 the intestines; among the largest, those of the skin, lungs, and 

 especially those of the medulla of bones. 



The size of capillaries varies necessarily in different animals in 

 relation to the size of their blood-corpuscles : thus, in the Proteus, 

 the capillary circulation can just be discerned with the naked eye. 



The form of the capillary network presents considerable variety 

 in the different tissues of the body : the varieties consist principally 

 of modifications of two chief kinds of mesh, the rounded and the 

 elongated. That kind in which the meshes or interspaces have a 



