THE BLOOD-VESSELS. • 293 



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 epithelia, are 

 developed quite independently. 



All the larger vessels down to those of j"' in diameter and 

 less, possess nutritive vessels as they are termed {vasa vasorum s. 

 nutrientia), derived from minute contiguous arteries and ramify- 

 ing 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 concurrent testimony of 

 many authors, also contains vessels, although in very small 

 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 in- 

 stances, in animals, divisions and free terminations of their fine 

 fibres may be perceived (vid. 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 de- 

 cisively 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 the vertebral canal, the vena cava, the jugular, iliac, 

 and crural veins, and in those of the liver by myself. They 

 are derived both from the sympathetic and the spinal nerves, 

 and with regard to their terminations have not yet been 

 investigated. According to Luschka, they would appear to 

 extend to the innermost vascular tunic ; but this I have not as 

 yet been successful in observing. 



