THE CAPILLARIES. 



127 



blood thrown back on the aortic valves, again recoiling. 

 Such a pulse is said to be dicrotous. 



92. The Capillaries are the smallest blood-vessels, those 

 through the walls of which materials pass to and from the 

 tissues, and so delicate that, as has already been pointed out, 

 even blood corpuscles are able, without injury to the walls, to 

 escape from them into the parts around. They vary from 

 ^(jo- to jj^Vo- i nc k i n diameter, and are arranged like the 

 meshes of a net. The meshes vary in size and form in 

 different localities ; for the most part they are polygonal ; in 

 the papillae of the skin they are in loops ; in muscle they 

 are oblong; and in the lung they are circular, with the 

 diameters of the circles little greater than the breadth of the 

 capillaries between which they lie. In some tissues they 

 can be seen under the microscope without previous prepara- 

 tion; and they exhibit the appearance of a homogeneous 

 membranous wall with oval nuclei imbedded in it, and 

 projecting to the outside. With the aid of a weak solution 

 of nitrate of silver, a delicate lining of epithelium, or endo- 

 thelium as it is sometimes called, is brought into view; but 

 it must not be supposed that the nuclei mentioned belong to 

 that lining. 



Fig. 69. CAPILLARIES, highly magnified. A, exhibits the nuclei; 

 B, the endothelmm as displayed by means of nitrate of silver. 



The blood can be seen circulating in the capillaries of the 

 web of a frog's foot, a tadpole's tail, or a bat's wing, without 



