THE BLOOD-VESSELS. 



309 



cause projections of it on the outei surface. The diameter of 

 the capillaries, in Man, varies from 0-002'" to 0006'" j and, for 

 the sake of description, they may be again subdivided into 

 finer , of 0002 — 0*003'", with few nuclei and thinner walls ; and 

 coarser, of 0*004 — 0006"', with a thicker membrane and nume- 

 rous nuclei; although in so doing it is not intended to draw 

 any distinct limitation between them. 



The capillaries, by their union, constitute the capillary 

 plexuses, retia capillaria, which have already been described in 



Fig. 287. 



detail, in speaking of the 

 individual organs and tis- 

 sues, and, consequently, here, 

 will be referred to only in 

 brief and general terms. The 

 forms presented in these 

 plexuses, which, notwith- 

 standing considerable diver- 

 sities, are constant in the 

 different organs, and accord- 

 ing to their similarity or 

 diversity, more or less cha- 

 racteristic, depend to some 

 extent upon the disposition 

 of the elementary parts, and 

 are also in some degree 

 dependent upon the energy 

 of the functions. With 

 respect to the former, there 

 are in many organs certain 

 tissues, into which vessels 

 never penetrate, — as the transversely striped muscular fibres, 

 the bundles of connective tissue, nerve-fibres, cells of all 

 kinds, gland-vesicles, and which, therefore, according to 

 their form, trace out definite courses for the capillaries, so that 



Fig. 287. Finest, vessels on the arterial side of the capillaries. 1. an artery of the 

 smallest size; 2, transitional vessel; 3, coarser capillary; 4, finer capillary: a, struc- 

 tureless coat, with a few nuclei, representing the t. adventitia ; b, nuclei of the mus- 

 cular fibre-cells; c, nuclei within the minute artery, probably still belonging to an 

 epithelium; d, nuclei of the capillaries and transitional vessels. From the human 

 brain, x 300 diam. 



