122 



STRUCTURE OF CAPILLARIES. 



membrane). Bundles of non-striped muscle, arranged longitudinally, 



occur in the adventitia of 

 the arteries of the penis, 

 and in the renal, splenic, 

 spermatic, iliac, hypogas- 

 tric, and superior mesen- 

 teric arteries. 



II. The Capillaries, while 

 retaining their diameter, 

 divide and reunite so as 

 to form net-works, whose 

 shape and arrangement 

 differ considerably in dif- 

 ferent tissues. The diame- 

 ter of the capillaries varies 



Fig. 41. 



considerably, but as a 



Capillaries-The outlines of the endothelial cells g eneral rule > ^ is such as 



marked off from each other by the cement to admit freely a single 



which is blackened by the action of silver row O f blood-corpuscles. 

 nitrate. The nuclei of the cells are obvious. 



Jn ^ retina and 



the diameter is 5-6 p, and in bone-marrow, liver, and choroid 

 10 20 IUL. The tubes consist of a single layer of transparent, ex- 

 cessively thin nucleated endothelial cells joined to each other by their 

 margins (Hoyer, Auerbach, Eberth, Aeby, 1865). 



[The nuclei contain a well-marked intra-nuclear plexus of fibrils, 

 like other nuclei.] The cells are more fusiform in the smaller capil- 

 laries and more polygonal in the larger. The body of the cells 

 presents the characters of very faintly refractive protoplasm, but it is 

 doubtful whether the body of the cell is endowed with the property of 

 contractility. 



Action of Silver Nitrate. If a dilute solution (-J per cent.) of 

 silver nitrate be injected into the blood-vessels, the cement substance 

 of the epithelium [and of the muscular fibres as well] is revealed by 

 the presence of the black " silver-lines." The blackened cement sub- 

 stance shows little specks and large black slits at different points. 

 It is not certain whether these are actual holes (J. Arnold) through 

 which colourless corpuscles may pass out of the vessels, or are merely 

 larger accumulations of the cement substance. 



[Arnold called these small areas in the black silver lines when they were large 

 stomata, and when small stigmata. They are most numerous after venous 

 congestion, and after the disturbances which follow inflammation of a part 

 (Cohnheim, Winiwarter). They are not always present. The existence of cement 

 substance between the cells may also be inferred from the fact that indigo- 



