SECT. 169.] THE SPLEEN — IN RUMINANTS, HORSE, ETC. 37 I 



well as the grey fibres of the nerve-trunks, is of an embryonic 

 character. Further investigation into the origin of these tissues 

 can alone determine the point. In the trunks of the splenic nerves 

 of the calf, before their entrance into the spleen, and in its interior, 

 there are seen numerous bifurcations of the dark-bordered primitive 

 nerve-tubes, both of the finer and of the coarser tubes. These I 

 have not yet succeeded in observing in man. 



With reference to the veins, many of the mammalia appear entirely to 

 agree with man ; others, on the other hand, as the horse, ox, sheep, pig, differ 

 very considerably. In these animals, a separate venous coat and vascular 

 sheaths arc only found at the commencement of the largest venous trunks, 

 whilst further inwards, they are only distinct on the side nearest the artery. 

 In all smaller veins, which run independently (without arteries), there is no 

 longer any trace of two envelopes to be found, and, indeed, these veins soon 

 appear to be simply excavations in the splenic substance, for a meshwork of 

 trabecule, with red spleen pulp in the meshes, may be seen on their walls. 

 They have, however, a perfectly smooth and shining surface, which is due to 

 a covering of spindle-shaped, pavement epithelial cells, o - oo5'" to o'oi'" in 

 diameter. This layer is only demonstrable by the microscope ; it corresponds 

 completely to that of the larger veins, only that in these animals it does not 

 lie upon a special wall, but immediately upon the splenic substance ; in other 

 w< trds, upon the trabecule and upon the delicate membranous tissue, bounding 

 the pulp between them. Such being the arrangement of the elements, we 

 are perfectly justified in speaking of venous sinuses, especially when we con- 

 sider that these veins, with these very attenuated walls, possess an enormous 

 width, and are perforated by innumerable veins, which discharge their contents 

 into them. These smaller veins themselves can be followed with the scissors 

 for some distance ; but I have not succeeded in demonstrating how they are 

 connected with the capillary network, although this network, arising from the 

 ordinary penicUli arteriarum, is itself very distinct. In truth, I do not think 

 it likely that the mode of connection between the capillaries and the veins 

 will ever be successfully made out ; for the finest veins are such delicate 

 canals, that they break up under the slightest mechanical force, as when 

 blown out or injected, and can no longer be seen by the microscope. On the 

 other hand, we are often able to trace veins onwards, until they become so 

 fine that it is impossible to speak of their beginning iu dilated extremities. 

 So that it would appear that a portion of the veins arises by capillaries of the 

 ordinary width, while it is probable that other very small veins open into 

 wide interstitial spaces, their walls of membrane and epithelium being con- 

 tinued in some way with the structureless film of the spaces. Small rows of 

 rounder epithelium, frequently found in the detritus of the pulp, probably 

 belong to these last-mentioned very small veins. 



The lymphatic vessels are stated by all authors to be very numerous in 

 malian animals. This is quite true for the rasa mperfieialia, which are 

 found in large numbers in the subserous tissue, where they anastomose in 

 various ways ; they are well seen in the calf. On the other hand, as I find, 

 the vasa profundiora are here scanty. Thus I have counted in the hilus of 

 the calf's spleen only four lymphatic trunks, having an aggregate diameter of 



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