STRUCTURE OF THE SPLEEN IN REPTILES. 351 



tissue containing numerous lymph corpuscles. At this part it 

 divides into fine branches, which run towards the centre of the 

 several follicles, where the smallest arterioles break up into a 

 very characteristic capillary plexus. This forms meshes of 

 O015 0'03 millimeter in width, which contain the paren- 

 chyma. The meshes are polygonal in form, strongly re- 

 sembling the capillary plexuses of the foetus ; the calibre of 

 the vessels exhibits, within a short space, variations of con- 

 siderable extent, and the wall, whilst it in part corresponds 

 precisely to that of ordinary capillaries, is in part constituted 

 of distinct nucleated cells, which are with difficulty, and only 

 through their somewhat more elongated form, distinguishable 

 from those of the adjacent parenchyma. Near the periphery of 

 the follicles the meshes of the capillary plexus diminish, whilst 

 the diameter of the vessels increases in size, and they at length 

 become continuous with a very close plexus of thin-walled 

 veins, which wind around the follicles. These veins, which in 

 some parts consist only of a thin connective-tissue layer con- 

 taining numerous cells, transmit their blood into larger 

 branches, lined with epithelium, and provided with layers of 

 muscular tissue, which partly run along the septa in the in- 

 terior of the organ, and partly in the deeper layers of the 

 capsule, to reach the point of entrance of the artery, by the 

 side of which they emerge from the organ as the splenic 

 veins. The fact that the walls of a portion of the capillaries 

 in the spleen of Ophidians very frequently present features 

 reminding the observer of their embryonic structure, naturally 

 suggests that besides a continuous new formation of lymph 

 corpuscles, a similar neoplastic formation of capillaries may 

 also take place, but what relation this process bears to the 

 function of the organ is not at present known. The plexus 

 of thin-walled veins which wind around the periphery of 

 the follicles resemble the lymph spaces that surround the 

 periphery of the follicles of the lymphatic glands. They repre- 

 sent at the same time the rudiment of a splenic pulp. If we 

 imagine the elements of the walls of these canals to become 

 developed into a plexiform tissue traversing the lumen of the 

 vessel, we shall obtain a tissue presenting the essential charac- 

 teristics of the splenic pulp, as it occurs in other vertebrate 



