186 



THE BLOOD VASCULAR SYSTEM 



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 

 through the capillary wall, by 

 diapedesis. The capillary wall 

 does not appear to be an inactive 

 factor in this process, for inert 

 pigment granules may also pene- 

 trate the wall of these vessels, 

 the endothelial cells immediately 

 closing the aperture which is 

 thus formed. Nevertheless, pure- 

 ly mechanical means, e.g., in- 

 creased blood-pressure, appear 

 also to favor this process. The 

 openings which are formed be- 

 tween the endothelial cells by 

 diapedesis of blood-cells are very 

 transitory; they are almost im- 

 mediately closed by the activity 

 of the endothelium. Such tran- 

 sitory breeches of the capillary 

 wall are termed stigmata. 



The capillaries branch and 

 anastomose with one another to 

 form networks, the outlines of 

 whose meshes vary according to 

 the tissue in which they occur. 



FIG. 206. Two SINUSOIDAL VESSELS FROM i n suc h tissues as muscle and 

 THE MEDULLA OF THE HUMAN ADRENAL. 

 Each contains the outline of a single red 



blood corpuscle for comparison of size. 



At a, a small vein is shown; it is filled with 



blood and possesses a much thicker wall 



than that of the sinusoids. Hematein and 



eosin. X 410. 



nerve they form elongated 

 meshes whose long axes are 

 parallel to those of the muscle 

 or nerve fibers; in the looser, 

 more areolar tissues they form 



large meshes of irregular form; 



while in the capillary membranes, as in the walls of the pulmonary 

 alveoli, they are disposed in a close net, the diameter of whose meshes 

 scarcely exceeds that of the capillaries. 



With but few exceptions capillaries occur in all the tissues of the 



