434 MANUAL OF HISTOLOGY. 



incomplete nature of the walls of the venous passages, these cells, con- 

 taining blood corpuscles in every stage of development, may make their 

 way into the stream occupying these passages, and thus become elements of 

 the splenic blood. 



Besides these. Funke and Koelliker mention, as farther elements of the 

 splenic pulp in young and sucking animals, other small yellowish nucleated 

 cells, which they hold to be young blood corpuscles in process of develop- 

 ment. Our own experience does not enable us to offer any remarks on 

 this point. 



232. 



We have still to consider the course of the blood-vessels and of the 

 lymphatics in the organ with which we are engaged, and to glance at the 

 arrangement of its nerves. 



Commencing with the veins, we find them liable to vary greatly in 

 different mammals. They are remarkable for their large calibre and great 

 distensibility, even when the distending force is but very small, a 

 peculiarity which explains the rapid physiological' and morbid congestions 

 with which this organ is affected. 



Among the ruminants, as, for instance, in the sheep and ox, the vena 

 lienalis enters the organ as a single trunk, parting with its aduentitia, 

 and soon after its media, to the surrounding connective-tissue sheath, and 

 then divides into wide branches, which send off a number of lateral twigs, 

 whose walls consist above of a very thin, membrane, so that these appear 

 on section as interstices in the parenchyma of the spleen. In their 

 further ramification, these vessels present an arborescent appearance, the 

 branches springing from them at right and acute angles, and no anasto- 

 mosis taking place among them. Thus the whole arrangement assumes a 

 peculiar character, from the fact that these venous ramifications (whose 

 calibre is remarkably great), breaking up rapidly into finer twigs, are 

 directed towards the numerous Malplgldan follicles in greater or less 

 number. All these venous tubules are possessed of walls of extreme 

 tenuity, but which are usually entire nevertheless. They consist, as a 

 rule, of a layer of fusiform cells, 0-0029-0'0079 mm. in breadth, and 

 0'0201-0'0501 mm. in length, whose elongated nuclei project to a small 

 extent above the surface of the cell. Externally the finer twigs are 

 enveloped in the reticular tissue of the pulp already mentioned.- 



Venous branches of this kind have been named by Billroth " capillary 

 veins," or " cavernous splenic veins." They are met with in all the 

 mammalia, though presenting much diversity as regards arrangement, by 

 which again the form of the pulp-cords is also modified. 



Whilst among the ruminants these cavernous veins pursue their 

 course with acute angled division, and without anastomosis, they break 

 up into branches among other animals, more or less, at right angles, in the 

 primary dendroid ramifications, and communication amongst the twigs of 

 the latter takes place, GO that eventually, and by degrees, a regular network 

 of like : sized venous canals, or more or less expanded passages, is formed. 

 This reticular arrangement is seen, for instance, in the spleen of the 

 rabbit, the Guinea-pig, the marmot, and likewise in man. In certain 

 cases the spleen of the infant displays with peculiar beauty this retiform 

 intercommunication of venous canals, and in such instances the lateral 

 twigs springing from ensheathed trunks assume almost immediately a 

 net-like character. I myselt was the first to establish, in the year 1860, 



