On their entry into the spleen, both arterial and venous branches receive as a covering a process of that " tunica propria" of the spleen which forms the vagince vasorum, previously described.
These are not alike in all animals : thus, for instance, they differ in man from those ex hibited by the higher brute mammalia— a fact which explains the various descriptions given by different authors. In man, the sheaths of the vessels form complete coats around them. A section made in the centre of the hilus, and continued through the spleen, exhibits them very distinctly as projections or processes of the tunica propria, and also allows their further circumstances to be seen. It is thus shown that arteries, veins, and nerves are thickly enclosed in these sheaths ; but in such wise that they are easily separated and isolated, especially in old, or macerated, or boiled spleens. The arteries and nerves allow of this more easily than the veins, which latter have a closer connection to these sheaths. It is further seen that not only are the trunks of entering and emerging vessels thus covered, but that their finer ramifications receive a similar clothing. The thickness of these sheaths is in the human subject by no means inconsiderable. As Giesker correctly states, they are at first exactly tEte thickness of the tunica propria, and retain the same thickness for a considerable distance, that is, as long as they clothe the main trunks of the vessels. On the branches which proceed laterally from these trunks, and on their fur ther extent, the sheaths become naturally finer, and gradually increase this fineness as the vessels become more minute, until finally, becoming very delicate, they lose themselves in the pulp of the spleen in the manner previously mentioned. The thickness of a sheath is always less than that of the coat of the artery which it incloses, and greater than that of the vein ; yet this does not hold good of vessels in all parts of their extent, since on the finest branches the sheaths are pro portionally sornewhat stronger than on the larger ones. As to the relations of the sheaths to the rest of the spleen substance, it must especially be considered that they do not lie free in the parenchyma of the organ, but are connected with the general trabecular network by means of balks which are given off from them: but these balks are not so numerous as different anatomists appear to think ; so that we are scarcely entitled to con sider with Giesker, that the whole trabecular network is formed out of this connection.
In other Mammalia, as in the horse, ass, ox, pig, sheep, Sze., the course of these sheaths differs in some respects from that seen in man. In the three latter animals, which in this respect are best known to me, no sheaths at all are found on the smaller veins, and on the larger they are chiefly found on that side on which the arteries and nerves which accompany them lie. Only the two primary trunks of the veins which proceed from the spleen have for a very short dis tance a complete sheath, while all the arte ries, even the finest, possess one; a con dition of which more will be said hereafter.
The minute structure of the sheaths of the vessels in man altogether correspond with that of the partitions ; and this holds good of animals generally. But I have
not been able to detect unstriped muscular fibre in the sheaths in all those cases in which I have found it in the trabeculm. In oxen this is especially the case; while, on the con trary, in pigs, &c., they are very plainly present.
Great difficulties oppose the inquiry con cerning the distribution of the vessels in the spleen itself: since, Istly, injection or in flation of the vessels gives little result on account of the delicacy of the organ ; and, 2dly, great difficulties are connected with the microscopic examination of the organ. What will be now adduced concerning it is espe cially the result of the latter method of in quiry, which, combined with fine preparations by the knife, has seemed to me to be the most fertile in results.
When the main branches of the splenic artery have entered into the spleen they lie in their sheaths, each in company with a vein, to which they are posterior and infe rior : they are in tolerably loose connection with the sheath, and not unfrequently they' take a serpentine course. In their further distribution they do not behave as arteries generally do, which continually give off' smaller branches, but they divide immediately into a quantity of different large and long branches in the manner of a shrub ; of these the larger branches go to the anterior, the smaller to the posterior, margin of the organ. Beside this, it is especially to be remarked of the arteries of the spleen, that their different branches form no anastomoses. Assolant tied a branch of the splenic artery in a living dog, and then allowed the spleen to return into the cavity of the belly. The dog died thirty hours after : much inflammation and exsudation of a bloody serous fluid was found in the belly, and the spleen was quite healthy; only the part cut off' from the circulation of the blood was gangrenous, and, as it were, separated from the sound part by a line of demarcation. In contrast to this, Heusinger tied all the branches of the splenic artery, one only excepted ; upon dissection, the whole spleen was found to be mortified, excepting the part in which the artery not deligated ramified. Also injections in an artery always return solely by the corresponding branch of vein ; and they only fill that region of the spleen in which the branch ramifies, never passing over into any other. I am unable froin my own experience to pass any judg ment upon these data, and will therefore not impugn them ; but I may be allowed to doubt whether the capillaries of the pulp are com pletely separated from each other, and am more inclined to believe that, in consequence of the anatomical circumstances of the pulp, such a separation; must be considered as im possible ; since in the spleen we have before us, not a gland with special lobes separated from each other, but a parenchyma every where united. The above results of deli gation and injection by no means necessarily imply an isolated course of the capillaries, and are fully. explained by the supposition that the arteries possess no anastomoses.