THE FLUIDS OF THE BODY 51 



In the wall of the alimentary canal it receives supplies via the 

 lymph. It drops them in the liver, its garde-manger, to pick 

 them up again as they are wanted. The torrent of lymph 

 which the thoracic duct discharges into the veins of the neck 

 conveys the fat which could not traverse the walls of the 

 capillary bloodvessels, and much of the reserve of food which 

 the blood had deposited in the liver. Only about one-quarter 

 of the fluid of the body (one-thirteenth of the body- weight) is 

 included within the blood-system ; but this enclosed fluid, 

 owing to the fact that it is kept in circulation by the heart, 

 replenishes and purifies the much larger quantity which does 

 not circulate. The unenclosed lymph has in particular situa- 

 tions a chemical composition which varies widely from that 

 of the blood. Imagine a marsh through which a river flows 

 the vast plains of water-plants on the Nile above Fashoda, 

 for example. There is a constant interchange between the 

 flowing water of the river and the stagnant water of the marsh. 

 In any given part of the marsh the quality of the water will 

 depend upon what it has been able to take from, and what it 

 has given back to, the river ; upon what the water-plants have 

 taken from it, and what they have added to it. Boats which 

 cannot penetrate the walls of reed keep to the open channel 

 of the Nile. Fish swim, now in the river, now in the narrow 

 passages and open pools of the marsh. So it is, in a way, with 

 the fluid in the spaces and cavities of the lymphatic system 

 and in the bloodvessels which traverse them, and with its 

 migratory inhabitants. In our extravagant analogy read 

 leucocytes for fish. Fish have two reasons for wandering 

 from river to marsh. Amongst the water-weeds they hunt for 

 food ; they seek quiet places in which to breed. In this matter 

 the analogy holds good. A leucocyte may be overtaken with 

 cell division anywhere in the blood-stream or in a lymph- 

 vessel. But cell division very rarely occurs except in certain 

 favoured spots. The breeding-places chosen by leucocytes are 

 sheltered situations in connective tissue where the blood- 

 supply is abundant, and the eligibility of such a spot is much 

 increased by its being near to a field where their services are 

 likely to be called for. The nests of connective tissue made 

 by the leucocytes are of three kinds, termed respectively diffuse 

 adenoid tissue, lymph-follicles, and lymphatic glands. The 



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