690 SPECIAL HISTOLOGY. 



They are everywhere composed of a single, structureless membrane, 

 with cell-nuclei, and are thus very essentially distinguished from the 

 larger vessels, although the transition from the one to the other is wholly 

 imperceptible ; so that at a certain point in the course of the vessels it 

 is quite impossible to detect the characters of either of the classes into 

 which Histology has been accustomed to divide them. Vessels of this 

 kind may be best described as venous and arterial transitionary vessels, 

 according as they lie on the one side or the other, and without any 

 further alteration of the common classification, may be referred to the 

 capillaries. 



The true capillaries, when more closely examined, exhibit the follow- 

 ing conditions: Their structureless membrane is perfectly clear and 

 transparent, sometimes delicate and presenting a simple contour, some- 

 times thicker (0-0008-0-001 of a line), and bordered by a double line. In 

 its microscopical reactions, it corresponds entirely with older cell mem- 

 branes and the sarcolemma of the transversely striped muscles (vid. 85), 

 and as regards its other properties, it is perfectly smooth on both 

 aspects, and notwithstanding its tenuity, tolerably resistant and elastic, 

 although very probably not contractile. It invariably presents a certain 

 number of elongated cell-nuclei, 0-003-0-004 of a line in size, which are 

 disposed with wide interspaces, usually alternately on opposite sides of 

 the vessel, sometimes more approximated, or in close contiguity, though 

 rarely in actual contact ; and, when the capillary tunic is thin, are 

 situated on its inner side, when thicker, within its substance, in such a 

 way, however, as not unfrequently to cause projections of it on the outer 

 surface. The diameter of the capillaries, in Man, varies from 0*002 to 

 0-006 of a line; and, for the sake of description, they may be again 

 subdivided into finer, of 0-002-0-003 of a line, with few nuclei and 

 thinner walls ; and coarser, of 0-004-0-006 of a line, with a thicker 

 membrane and numerous 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 detail, in speak- 

 ing of the individual organs and tissues, and, consequently, here, 

 will be referred to only in brief and general terms. The forms pre- 

 sented in these plexuses, which, notwithstanding considerable diver- 

 sities, are constant in the different organs, and according to their simi- 

 larity or diversity, more or less characteristic, depend to some ex- 

 tent 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 muscu- 

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

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



