THE FLUIDS OF THE BODY 79 



tinged man's mental outlook, preventing him from seeing 

 objects in their natural brightness ; but the spleen made black 

 bile, which, mounting to the brain, displayed its malign 

 influence upon the action of that organ, as, or in, the worst 

 of humours. 



The spleen is invested with a capsule of no great toughness. 

 Inside the capsule is " spleen pulp." When the fresh organ 

 is cut across, it is seen that, although most of the pulp is of 

 the colour of dark venous blood, it is mottled with light 

 patches. In some animals the cat, for example these 

 whitish patches are small round spots, regularly arranged at a 

 certain distance from the capsule. The distinction into " red 

 pulp " and " white pulp " marks a division into two kinds 

 of tissue with entirely different functions. The white pulp is 

 lymphoid tissue, lymph-follicles developed in the outer or 

 connective-tissue coat of the branches of the splenic artery. 

 Its function is to make lymphocytes, of which, for reasons 

 which will shortly appear, the spleen needs an abundant 

 supply. The constitution of the red pulp is entirely different, 

 and peculiar to the spleen. The branches of the splenic artery 

 divide in the usual way into smaller and still smaller twigs 

 until the finest arterioles are reached ; but these arterioles do 

 not give rise to capillary vessels. At the point at which in 

 any other organ their branches would attain the calibre of 

 capillaries, the connective-tissue cells which make their walls 

 scatter into a reticulum. They are no longer tiles with 

 closely fitting, sinuous, dovetailed borders, but stellate cells 

 with long delicate processes uniting to constitute a network. 

 The blood which the arterioles bring to the pulp is not con- 

 ducted by closed capillary vessels across the pulp to the 

 commencing splenic veins. It falls into the general sponge- 

 work. The venules commence exactly in the same way as the 

 arterioles end. Stellate connective-tissue cells become flat 

 tiles placed edge to edge. The endothelium of an arteriole 

 might be likened to a column of men marching shoulder to 

 shoulder, three or four abreast ; the connective tissue of the 

 pulp, to a crowd in an open place. The column breaks up into 

 a crowd. On the other side the crowd falls into rank as 

 the endothelium of veins. The capsule and the red pulp are 

 largely composed of muscle-fibres. These relax and contract 



