428 



MANUAL OF HISTOLOGY. 



Fig. 418. From the spleen of a pig. a, an arterial twig 

 invested with its sheath, showing its twigs, 6, and 

 attached Malpighian corpuscles, c. 



the interior of the organ its own individuality as regards its ramifications. 

 Soon after there commences a most extensive division and subdivision of 



the vessels, until, finally, the 

 latter, greatly diminished in 

 size, form a series of terminal 

 groups, which have been long 

 compared to the hairs of a paint- 

 brush. But a more appropriate 

 comparison has been made be- 

 tween these " Penicilli " and 

 the branches of a willow tree 

 divested of its leaves. Fig. 

 418 gives a tolerable represen- 

 tation of the arrangement re- 

 ferred to. 



Drawing such a branch out 

 of the tissue of the spleen, we 

 may recognise on it the follicles 

 of which we have been speak- 

 ing. They are of a whitish 

 colour, and hang on the fine 

 arterial twigs like grapes on 

 their stalk. They are either 

 attached by their border to the 

 artery, or the latter traverse 

 their interior; or, finally, the 

 angles of division of such a series of branches may be surrounded by 

 numbers of them for a considerable distance. In form they are some- 

 times spheroidal, sometimes more or less elongated. 



Such spleen corpuscles are to be found in all the mammalia, although 

 presenting much variety. In the human organ, however, they are less 

 distinct as a rule than elsewhere; and in bodies which have suffered from 

 protracted illnesses they were formerly supposed not to exist, while in 

 those in which death had occurred suddenly they were said to be always 

 recognisable, even without the microscope ; as also in youthful corpses 

 (von Hessling). For this reason they were looked upon even years ago 

 as integrant portions of the human spleen. 



If we follow' up the disposal of the vessels commencing at the 

 hilus, we soon remark that it is liable to vary greatly in different animals. 

 The sheaths of these tubes also are no less subject to variation. Though 

 very imperfectly developed in the Guinea pig, rabbit, squirrel, and mar- 

 mot, they attain a high degree of development in other animals, as, for 

 instance, in the dog and cat. There the arteries enter the spleen in 

 several branches, each of the latter accompanied by a vein and one or 

 two nerves. Both artery and vein while passing in receive a sheath, but 

 not in the same way. Around the artery the latter is loose, and only 

 runs for a short distance unchanged, undergoing rapidiy a peculiar 

 lymphoid transformation. The vein, on the contrary, is accompanied for 

 a much greater distance by a tight investment, closely united to its walls. 

 On the smaller venous twigs the latter resolves itself into a few bands of 

 connective-tissue, which sink into the septa of the spleen. Deviations 

 from this general plan are to be seen in the ruminants and the pig. 



In man the arteries and veins arrive in the spleen, already divided into 



