Capillary blood-vessels from mesentery 

 of young dog : n, the capillaries, with 

 the nuclei of the endothelial plates, lying 

 within the connective tissue (g). 



THE CIRCULATORY SYSTEM. gg 



The capillaries form rich net-works in almost all tissues and organs, 

 the principal localities where these vessels are wanting being epi- 

 thelium, the hairs, the nails, teeth, cartilage, the cornea, the crys- 

 talline lens, and certain parts of the nervous system. 



The capillary net-works vary in the size both of the meshes and 

 of the constituent vessels. The average diameter of the capillaries 

 is 7-10 (j. ; the smallest are found in the brain, retina, and muscle ; 

 the largest in bone-marrow, dentinal 

 pulp, and the liver. The closest meshes 

 are found in the air-vesicles of the lungs, 

 the choroid, the liver, and other glands ; 

 the widest in the serous membranes, 

 tendon, etc. Young tissues are more 

 richly supplied than old ones. 



The capillaries consist of a single 

 layer of endothelial cells, united by 

 intercellular cement-substance ; they 

 are, consequently, protoplasmic tubes 

 of high vitality, admirably designed to 

 facilitate the interchanges constituting 



nutrition. After staining with silver the endothelial plates are seen 

 as extended spindle-cells, united by irregular lines of darkened 

 cement-substance ; at the points where the vessels branch, irregular 

 triangular cells are not infrequently seen. In such preparations, 

 likewise, along the lines of union or at the juncture of several plates, 

 irregular darkened areas the stigmata may be observed ; these 

 are probably minute spaces occupied by stained albuminous sub- 

 stances ; these areas are supposed to aid the diapedesis or trans- 

 migration of the blood-cells. 



Some capillaries are invested by an imperfect adventitious coat, 

 formed by a net-work of surrounding branched connective-tissue 

 cells, and resembling the reticulum present in lymphoid tissue. The 

 intimate relation existing between the endothelium of the vessels 

 and the surrounding connective-tissue corpuscles is well exhibited in 

 young growing tissues, as the omentum. 



The peculiarities distinguishing the capillaries from the small 

 "capillary" arteries or veins consist not so much in the size of the 

 vessels for the capillaries may have absolutely the greater calibre 

 as in the character of their walls. The true capillary possesses no 

 muscle-cells, these first appearing in irregular groups beyond the 

 limits of the capillary vessel ; in those cases where, as in certain 

 veins, muscular tissue is wanting, the character of the adventitia of 

 the vein will aid in determining the character of the vessel. 



Small blood-vessels the vasa vasorum provide for the nutri- 



