THE BLOODVESSELS. 677 



exists in all vessels, may almost always be pretty readily broken up 

 into its elements, and like other simple cpithelia, is not subject to any 

 constant detachment arid restoration. WithJjUrnak, \ve might describe 

 the epithelium as the cellular membrane of the vessels, since it differs 

 from other epithelia in this respect, that in the large vessels, it is often 

 continuous with the striped lamellce, without any line of demarcation, 

 so that frequently it cannot be said where the one ceases and the others 

 commence ; but I should myself rather be inclined to retain the old 

 name, both because the innermost cellular layer of the vessels presents 

 in all respects the relations of a simple epithelium, and is, in many 

 situations (heart, smaller vessels) abruptly defined from the deeper 

 tissues. Even the circumstance particularly adduced by Ilemak, that 

 the vascular epithelium does not proceed from the embryonic epithelial 

 membrane, is not with me decisive as to the propriety of separating it 

 from the other epithelia, inasmuch as the investments of the serous 

 sacs and synovial capsules, which no one will be disposed to separate 

 from the cpithelia, are developed quite independently. 



All the larger vessels down to those J a line in diameter and less, 

 possess nutritive vessels as they are termed (vasa vasorum s. nutrientia), 

 derived from minute contiguous arteries, and ramifying chiefly in' the 

 t. adventitia, in which they constitute a rich capillary plexus with 

 somewhat rounded meshes, from which again the veins accompanying 

 the arteries arise, and which, in the case of the venous vasa vasorum, 

 pour their blood directly into the vein to which they belong. The t. 

 media of the larger arteries and veins, according to the pncurrent 

 testimony of many authors, also contains vessels, although in very 

 11 number, and only in the outer layers; whilst the internal layers 

 and the t. intima have always appeared to me to contain none at all; 

 but even in these situations some observers would seem to have noticed 

 vessels (in the Ox, the vena cava inferior is richly provided with vessels 

 up to the t. intima). Nerves derived from the sympathetic and spinal 

 nerves may readily be seen going to many arteries, but frequently 

 appear merely to accompany them. Where they enter the coats of a 

 vessel they run only within the t. adventitia, and in favorable instances, 

 in animals, divisions and free terminations of their fine fibres may be 

 perceived (vide "Mikros. Anat.," II., 1, p. 532-33). Many arteries are 

 wholly without nerves, as those of the cerebral and spinal substance, of 

 the t. chorioidea, the placenta, as well as many arteries of muscles, 

 glands, and membranes, whence it is obvious, that nerves are not as 

 requisite for them as we have usually been inclined to believe. This 

 may be said more decisively with respect to the veins, as it is only in 

 the larger ones that a few fine nerves can be demonstrated. Such have 

 been observed by Luschka in the sinuses of the dura mater, the veins of 



