Giesker at least partially agrees with this statement when he says that the pulp con sists of nothing but the minutest arteries and veins united by fibrous tissue. The sheaths of the vessels above described are just as much more delicate as are the vessels themselves, and they are finally lost as distinct coats on the capillaries ; here they form delicate fibrous membranes which connect the capillaries to gether, and under this form they pass through the whole of the pulp.
As to the veins, I must first, with Giesker, express myself in the most decided rnanner against all the more ancient and modern ana tomists who suppose and describe venous spaces (sinus venosi) in the hunian spleen. I have bestowed the greatest attention to the dilated commencements of the veins in ques tion, and it was only my own researches that led rne to renounce the opinion that these dilatations really exist ; indeed I have never been able to discover anything special or ex traordinary about these veins. Firstly, as to the larger veins, which are as yet accom panied by the arteries, there is nothing very remarkable about thetn, with the exception of their considerable size, which has been already mentioned. They all have a membrane which is continuous with that of the smaller veins, and is least separable on that side with which the artery is in contact ; this membrane is only distinguishable from the sheath of the vessels by its greater delicacy, and in com pany with this sheath it gradually diminishes its thickness. Orifices of the smaller veins, constituting the so-called stigmata Malpighi, are present in very small numbers in the larger veins ; while, on the other hand, they are somewhat more frequent in the smaller of the vessels in question. When the veins leave the arteries and pursue their way alone, they vary in some respects from this descrip tion, although not so considerably as might he imagined from the delineations which have been given of them. In the first place, the character of the branchings is peculiar, since from hence onwards, and so much the more frequently the smaller the veins become, branches are given off from the veins on all sides at very nearly right angles, and the open mouths of these ramifications are seen from within as numerous round or oval orifices lying very closely to each other. In the second place, the membranes of these veins gradually become thinner and thinner, and at the same time are blended with the similarly attenuated sheaths, so that both constitute only one delicate membrane, which is never theless everywhere demonstrable even in the smallest vessels which can be isolated, and which everywhere exhibits itself without any interruption as a perfectly continuous mem brane. Dilatations or pouchings can no where be seen, either in the course of the isolated veins or in their smallest branches ; only it must be added, that the narrowing of their calibre occurs much more slowly than in the arteries. As to the beginnings of the veins, and their connection with the ca pillaries, I have not been able to detect any thing more than what one sees elsewhere ; namely, that by a constant simplification and attenuation of their structure, the veins finally pass into capillaries. Here also no traces of dilatations are visible, of whatever kind these dilatations might be imagined to be ; and there is just as little appearance of any other pecu liarity.
As regards the brute mamnialia, many of them certainly correspond in a very considerable de gree with man, in respect of the condition of these vessels ; but my researches do not ex tend sufficiently to enable me to express myself decisively on this point. While, on the other band, some, as the horse, ass, ox, pig, and sheep, exhibit essential differences. In the latter animal, which I have examined the most carefully, the following deviations are pre sent. The arteries differ little from those of man, only they separate earlier from the veins to pursue their isolated course. In most other respects they behave precisely as J. Miiller has described them, and as I have also spoken of them in man ; only I cannot corroborate the statement of Midler, that the sheaths of the smaller arteries are equal in strength to those of the greater. The rami
fications which reach the Malpighian cor puscles measure from 1 to 11-100ths ofa line in diameter ; they then course in the pulp, form very beautiful tufts, and finally capillaries, of which the smallest measure from 3 to 4 1000ths of a line. But in contrast to this, the veins exhibit very essential differences. In the first place, a special membrane and sheath are only found in the largest venous trunks, and even here they only extend a short dis tance around the circumference of the vessel ; while more deeply in the spleen they only lie upon the side where the artery and nerve are attached to the vein. In all the smaller veins which are no longer accompanied by arteries, there is no trace of these two membranes to be seen ; and not only is this the case, but the inode in which the precise limit of the venous canal is indicated is also very extra ordinary. The vein appears to be formed in the first instance by the strong anastomosing trabeculx, and soon afterwards it seems com posed simply of delicate fibres and red sub stance deposited between them, a structure which continues even into the large venous trunks. They thus distinguish themselves at the first glance as excavations in the paren chyma of the spleen, which are devoid of walls. Nevertheless, by a more careful ex amination of the red limits of these veins, one may verify their smooth and shining appear ances, a circumstance which is significant of the existence of a delicate membranous cover ing ; and, in point of fact, microscopic inves tigation proves the existence of an epithe lium, which every where clothes this surface, and conaists of fusiform or more spherical cells, of to 1-100th °fa line in diameter, with roundish or elongated nuclei of 3 to 5 1000ths of a line in size (fig. 534.). This epi thelium altogether corresponds with that which covers the part of the veins possessing a visible membrane ; but in the vessels of which I am speaking, it is placed in part imme diately on the trabeculm, in part upon a deli cate fibrous membrane limiting that part of the pulp which bounds the veins. In con sequence of what has been said, the greater number of the splenic veins of the ox must be likened in respect of their structure to the spaces in the corpora cavernosa penis, and to the sinuses of the dura mater ; since, in stead of the venous membranes elsewhere present, they possess only the " tunzea =lima " in the shape of a delicate epithelium. So that one may speak of them as " venous sinuses," and the more correctly, if it be considered that these veins, ahnost devoid of walls, possess a colossal width, and are every where rendered quite cribriform by largOr and smaller veins opemng into their interior ; which smaller veins may themselves be traced by their great width for a considerable dis tance. How these smaller veins are con nected with the very distinct capillury net work of the pulp, I have not been able to find out ; and I do not believe that either injec tion or inflation of the vein, or a microscopic examination, will ever give any definite con clusion hereto. For these vessels, often pos sessing but a few little trabeculw for their coats, are of such a delicate texture, that they tear by the slightest mechanical force, while by the microscope they cannot be distinguished from the surrounding constituents of the pulp. Yet thus much one may see, that the veins gradually become very small,—so small, that it is quite impossible to talk of their commen cing as dilated spaces. For my own part, I am convinced that a similar communication obtains between the veins and capillaries of oxen as of men ; and that the only possible dift"erence is, that the veins here possess only an epithelium, and must therefore be con nected with the capillaries in a somewhat dif ferent way. I will yet further add, that in microscopic examination of the pulp of ani mals, skeins of epithelium are not unfre quently found, consisting of roundish cells, as it were, fused together : these can only come from the small venous trunks.