498 CAPILLARIES. [SECT. 2l6. 



internal periosteum. 4. The venous spaces of the corpora ca- 

 vernosa (see § 200), and of the spleen of certain mammalia (see 

 § 169). 5. The veins of the retina. — The valves of the veins 

 chiefly consist of distinct connective tissue, which runs parallel 

 to their free border, and contains numerous elongated nuclei, 

 as well as scattered elastic fibres, wavy and mostly free, some of 

 them being of the thicker variety. On the surface, there is 

 found either a simple epithelium with short cells, or, in addition, 

 a very fine elastic network beneath the cells, disposed principally 

 in the longitudinal direction. The valves, accordingly, may be 

 regarded as continuations of the middle and inner coats, although, 

 as far as I have seen, muscular fibres are wanting in them. 

 Wahlgren, indeed, believes he has seen them in large valves, but 

 Remak regards the appearance of them as caused by the outer wall 

 of the vein in the neighbourhood of the valves, where the other 

 two coats happen to be thin. 



§ 216. Capillaries, Vasa Capillaria. — "With the single exception 

 of the cavernous structures in the sexual organs and in the uterine 

 placenta, all the arteries and veins of the human body are connected 

 by abundant networks of microscopically fine vessels, which, on 

 account of their narrow calibre, have received the above desig- 

 nation. They everywhere consist of a siugle, structureless coat 

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

 the larger vessels ; still the transition to the arteries on the one 

 side and to the veins on the other is quite imperceptible, so that at 

 a certain point it is quite impossible to recognise the characters 

 assigned by the histologist as the distinguishing marks of the one 

 or the other division of vessels; such vessels, therefore, may be 

 best designated, according to the side on which they lie, as venous 

 or arterial vessels of transition, and without further alteration in 

 the general arrangement, we may rank them with the capillaries. 



The proper capillaries, when narrowly examined, present the 

 following conditions. Their structureless coat is perfectly bright 

 and clear, sometimes delicate and bounded by a simple contour, 

 sometimes thicker, up to o"OOo8"' and 0001'", and with a distinct 

 double contour. It completely agrees in its microscopical reactions 

 with old cell-membranes and with the sarcolemma of the trans- 

 versely striated muscular fibres (§ 85) ; and with regard to its 

 other properties, it is perfectly smooth internally and externally, 

 tolerably resistant and elastic, notwithstanding its fineness, yet, in 

 all probability, it is not contractile. It always, and without ex- 



