CAPILLARIES 91 



is formed by a layer of endothelial cells which on the one hand is 

 continuous with the endothelial lining of the arteries, on the other 

 hand with that of the veins. 



As a rule there are neither muscle fibres nor connective tissue 

 in the wall of the true capillaries ; occasionally, however, very fine 

 isolated circumferential elastic fibres encircle the endothelial tube. 

 In the minute arfcerioles and venules, which are about to terminate 

 in or take origin from the true capillaries and which have been 

 described as precapillary arterioles and venules, a very thin layer 

 of muscle fibres or of connective tissue is added to the endothelial 

 wall of the capillary. On the arterial side the muscle is the first 

 tissue to be thus added, on the venous side the fibrous connective 

 tissue is the first to appear. 



The endothelium of the capillary wall consists of flattened 

 plate-like cells which are joined edge to edge by cement sub- 



FIG. 92. CAPILLARY VESSEL OF THE FROG'S MESENTERY. 



Treated with nitrate of silver to show the outlines of the endothelial cells. Highly 

 magnified. (After Kan vier.) 



stance. These cells are somewhat elongated in the axis of the 

 vessel, the shape of the cell, as in the arteries and veins, depend- 

 ing upon the size of the vessel the smaller the vessel the more 

 elongated its endothelial cells. The margins of these cells are 

 extremely irregular, hence they present a wavy or serrated outline. 

 Although the endothelial cells of the capillary wall appear to 

 be firmly united to one another, yet they are capable of being 

 separated sufficiently to permit the ready passage of white blood 

 cells though the capillary wall, by means of diapedesis. The 

 capillary wall does not appear to be an inactive factor in this 

 process, for inert pigment granules may also penetrate the wall 

 of these vessels, the endothelial cells immediately closing the 

 aperture which is thus formed. Nevertheless, purely mechanical 

 means, e. g., increased blood pressure, appear also to favor this 

 process. The openings which are formed between the endothelial 

 cells by the process of diapedesis are very transitory ; they are 

 almost immediately closed by the activity of the endothelium. 

 Such transitory breeches of the capillary wall are termed stigmata. 



