Vasa vasorunz.— The coats of all blood ves sels, except those of very small size, are sup plied with arteries and veins especially devoted to their nutrition, called " vasa vasorurn." According to Henle, vessels (veins and arteries in common) receive vasa vasorum, though themselves not larger than of an inch in diameter, and sometimes even smaller. The coats of capillaries and those of the smallest vessels are not thus nourished by a separate vascular supply.
The nutrient arteries of the venous coats are derived from the small arterial trunks in the vicinity of the vessel-- ; they do not come directly from larger arterial trunks, but from smaller vessels; and the source of the vascular supply is determined by the particular neigh bourhood of the vein—the same vessel having vasa vasorum from different sources as it passes along different regions : thus, the versa azygos is nourished by the intercostal, pericardial, and oesophageal, arteries, according to its various relations as it proceeds along its course. The arteries supplying the tunics of a vein, appear to go promiscuously to the vessel and the adjacent tissues, sending some of its ramules to the areolar tissue, nerves, fat, &c., in the neighbourhood, and others to the walls of the vessel. In the accompanying figure, repre senting a vein from the cesophagus of an eel, magnified with low power, the vasa vasorum are seen to supply, indiscriminately, the vessel's coats and some small pellets of adipose tissue which were near it. The arteries, nourishing a vein, ramify and divide principally upon, and among, its cellular coat, and form an elaborate plexus, with meshes having a general longi tudinal direction, as seen in the figure ; which, though drawn from a fish, sufficiently indicates the condition as found in man and mammals. The arrangement of these capillaries is, how ever, subject to variety : in the vena cava of the cod, I have seen them long, straight, even, and perfectly parallel, with scarcely any trans verse branching or anastomosis. The little -venous trunks of the vasa vasorum usually open directly into the cavity of the vein, among whose tunics they have previously ramified, and their course is quite independent of the corresponding arteries. (Henle.) The vasa vasorum principally exist in the areolar tunic, but Henle states that they are to be found abundantly among the annular fibres of the veins.% The internal longitu
dinal coat is, in all cases, extra-vascular.
Nerves of veins.— As far as our knowledge hitherto goes, veins differ remarkably from arteries in rarely being made the support of nerves, and in seldom receiving any distribu tion of them to their coats. It is a remark able circumstance that veins appear, with slight exceptions, to be separated from nerves. The exceptions to this rule are few and in considerable.
The inferior vena cava receives some small filaments from the diaphragmatic ganglion of the sympathetic, just below the diaphragm. The vena porta supports and is entwined by the branches of the hepatic plexus, which it conducts to the liver. Pappenheim describes some nerves as being distributed to the cere bral veins. Bidder states that he has traced some filaments of the fourth cerebral nerve to the lateral sinus : and Wrisberg describes a nervous plexus as surrounding the facial vein, which he ingeniously imagines may cause contractions of the vein, and thereby pro duce that congestion of the face constituting blushing.
In the cod-fish (Gadus morrhua), I have traced nerves from the auricle down upon the venous sinus immediately below it : these formed loops in the walls of the sinus, and appeared to return to the heart. The nerves in question are doubtless a continuation of the cardiac nerves ; and, in all probability, bear the same relation to the rhythmical action of this pulsating cavity, which the other car diac nerves do to the cavities of the heart itself.
It is very probable that some such nervous supply is furnished to those parts of the veins of man and mammals, near the heart, in which cardiac muscle is found.
It may, perhaps, be still a question whether the almost complete non-nervous character of veins is absolutely correct, and whether the paucity or tenuity of the nerves, supplied to veins in general, be not the reason why they have as yet eluded observation ; for analogy scarcely justifies the idea of organs possessing muscular tissue and still being destitute of nerves. This opinion is strengthened by the fact that arteries, having the same sort of muscularity as veins, though in larger amount, are regularly supplied with nerves.