308 SPECIAL HISTOLOGY. 



may, therefore, be regarded as continuations of the middle and 

 internal tunics, although muscular fibres, so far as I have seen, 

 are wanting in them (Wahlgren states that he has found such 

 fibres in the larger valves) . 



§ 217. 



Capillaries (vasa capillaria). — With the solitary exception 

 of the corpora cavernosa of the sexual organs, and of the 

 uterine placenta, the arteries and veins, in Man, are universally 

 connected by the intervention of a rich plexus of microsco- 

 pically fine vessels, which, on account of their slender dimen- 

 sions, have been designated under the above name. 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 cha- 

 racters 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 following conditions. Their structureless membrane is 

 perfectly clear and transparent, sometimes delicate and pre- 

 senting a simple contour, sometimes thicker (0'0008 — 0001'"), 

 and bordered by a double line. In its microscopical reactions, 

 it corresponds entirely with older cell-membranes 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 contrac- 

 tile. It invariably presents a certain number of elongated 

 cell-nuclei, 0003 — 0-004'" 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 



