80 THE CIRCULATORY SYSTEM. 



spaces formed by separation of the layers of the dura 

 mater. 



Some veins are devoid of muscle-tissue. 



Capillaries : The capillaries are minute tubes formed of a 

 single layer of elongated flat squamous nucleated endothelium- 

 cells, with their long axes arranged longitudinal to the course 

 of the vessels. They are, therefore, continuations of the 

 endothelial linings of the arteries and veins, but divested of 

 all other coverings, except that whatever traces of connective 

 tissue may surround the endothelial tubes may be regarded as 

 vestiges of the adventitia. 



At the junction of the endothelial cells in places are small 

 points, which are perhaps stomata or openings, by which the 

 escape or migration of leukocytes through the wall of the capil- 

 lary is effected or facilitated. 



The capillaries form a system of exceedingly numerous short 

 minute passages connecting the terminal arteries on the one 

 side with the initial veins on the other. They branch and 

 anastomose so as to form reticula or networks, varying in the 

 closeness, form, and arrangement of the meshes in different 

 organs and situations, according to the blood supply locally 

 required. The capillaries lie in intimate relation with all the 

 tissues, and nearly all parts of the body are closely permeated 

 by these channels. From the capillaries the blood accom- 

 plishes its metabolic and nutritional action on the body-tissues. 

 The capillaries are therefore the ultimate active functional 

 elements of the circulatory system, to which all the rest of 

 the circulatory apparatus serves a subordinate and tributary 

 purpose. 



The thin walls of the capillaries are highly endowed with 

 the vital properties of protoplasm, and are well fitted for the 

 exosmosis of nutrient fluids ; the endosmosis of carbon dioxide 

 and other waste products of tissue-katabolism ; and the passage 

 of leukocytes through them. 



Vascular sinuses : In some situations the vascular passages 

 are dilated or distended in the form of cavernous spaces or 

 sinuses (often called venous sinuses), lined with endothelium 

 and communicating with arteries and veins, and with one 



