286 SPECIAL ANATOMY. 



granular membrane of pavement epithelium, which is also seen in the most 

 delicate capillaries. 



b. The striated or fenestrated membrane is delicate, clear, fragile, and 

 rolls up at the corners. 



c. The longitudinal, fibrous membrane is distinctly seen in large vessels. 

 It is, like a. and b., formed of oval cell nuclei or fibres, which are placed lon- 

 gitudinally, in which direction the membrane may be torn off; and it was 

 formerly considered, with a. and b. as the most internal layer of the vessels. 



d. The circular, fibrous coat (so-called tunica med.) is thick, contracting 

 upon certain stimuli (falsely, tun. elastica); consists of short parallel fibres, 

 which are soluble in acetic acid, and of dark striae, which are only rendered 

 transparent by the acid ; it is fragile, and tears off transversely. 



e. The elastic coat is only present in the larger arteries and-veins; is 

 white, thinner than d. ; is neither soluble in, nor rendered transparent by ace- 

 tic acid ; tears off neither in the longitudinal nor transverse direction, and 

 consists of very branched, often reteform, fibres. 



f. The external tunic (tun. adventitia) forms with e. the hitherto so-called 

 tunica externa; it consists of fibro-cellular tissue, with oval nuclei in a longi- 

 tudinal direction, and passes from the larger vessels into the surrounding 

 amorphous, fibro-cellular, or uniting tissue. 



The nutrition of the larger vessels (above half a line diam.) is supplied by 

 small arterial twigs (vasa vasorum), which do not come immediately from the 

 cavity of the vessel for which they are destined, whilst the venous ramusculi 

 open into those veins, from the coats of which the blood collects. The vasa 

 vasorum themselves, as well as the capillary vessels, are nourished by the 

 blood circulating in them. 



Nerves : (nervi vasorum) the vessels obtain their supply from the N. Sym- 

 pathicus. 



523. I. Capillary vessels, vasa capillaria, 



are the blood-vessels intermediate between the arteries and veins, the 

 branches of which form a net-work with each other, and tolerably equally- 

 sized meshes. In these meshes is placed the peculiar substance of the tissue, 

 and it receives its nutrition from the blood of the vessels (metamorphosis of 

 matter). The blood circulating in them is neither arterial nor venous ; in the 

 finest vessels it appears colourless ; it generally flows in a direction from the 

 arteries, which here terminate, to the veins, which commence at this point. 

 Ii is impossible to draw a definite limit between the terminations of the arte- 

 ries and the commencement of the veins. Structure. In the parietes of the 

 capillary vessels (which are not merely canals in the parenchyma of organs), 

 pavement epithelium, and, most externally, a tunic of areolar tissue, has been 

 found. 



The forms of their network vary according to the diameters of the vessels 

 (the finest in the brain = O'OOS of a line ; the largest in the medulla of the 

 bonesr== 0*010. of a line); according to the breadth of the meshes, which is so 



