THE BLOODVESSELS CAPILLARIES. 505 



to T2 J^o of an inch), and bordered by a double line. It corresponds 

 chemically with the myolemma of the strfated muscular fibres; is 

 perfectly smooth on both sides, and tolerably resistant and elastic, 

 though probably not contractile. (Kolliker.) The nuclei are elon- 

 gated, 4 oVo to 3 inrtf of an inch in diameter, and usually alternate 

 on opposite sides of the vessel, though rarely in contact with each 

 other. When the capillary wall is thick they are lodged in its 

 substance, though sometimes projecting externally ; when thinner, 

 they are situated on its inner side. (Fig. 337.) Their diameter 

 varies from goVo to 2oVo f an inch. 



The union of the capillaries constitutes a capillary plexus, already 

 described, in connection with the different tissues and organs. As 

 the histological elements of the various tissues are not themselves 

 penetrated by capillaries, but the latter are merely distributed 

 among the former, the capillary plexus of each tissue and organ 

 will assume a form more or less peculiar. Generally, indeed, a tissue 

 or organ may be at once recognized by its capillary network alone. 

 It is closest in the secreting organs (especially the liver, lungs, 

 and kidneys) (Figs. 406, 375) ; next in the skin, and mucous mem- 

 branes (Figs. 322 and 353); and is much wider in organs receiving 

 blood only for their own nutrition, as the muscles, nerves, organs 

 of sense, serous membranes, tendons, and bones. (Figs. 266, 176, &c.) 

 The muscles and the gray nerve-substance are, however, more 

 abundantly supplied than the other parts just mentioned. Their 

 diameter also varies directly with the closeness of the plexus; it 

 being ^-9 to 2*W of an inch in the glands, ^ to ^Vu i n the 

 skin and mucous membranes, and thinnest and smallest (goW to 

 4 oVo^ f an inch) in the nerves, muscles, retina, and the patches of 

 Peyer. In the compact bone structure, however, though no longer 

 having, in all respects, the structure of capillaries, they attain to 

 the diameter of jg^ to T2 Vtr f an inch (p. 337, 2). 



On tracing the capillaries towards the arteries, the nuclei become 

 more closely placed, and a structureless tunica adventitia, and soli- 

 tary muscular cells, appear externally ; and becoming y? 1 ^ of an 

 inch in diameter, they exhibit the aspect of the finest arteries. 

 Afterwards the nuclei seem to be replaced by epithelium, and the 

 membrane of the capillary either ceases, or is continuous with the 

 fenestrated layer of the artery. (Fig. 337, B.) 



The capillaries merge in the opposite direction into the veins by 

 less characteristic changes. On this side an external nucleated 



