50 THE BODY AT WORK 



the apposed surfaces of the endothelium which lines their 

 walls and covers the organs which they contain. There 

 is no fluid in them which can be collected and labelled " peri- 

 toneal " or " pleural " fluid. The purpose of the spaces is to 

 allow of movement without friction in the one case of the 

 intestines, in the other of the lungs. It is possible to 

 take a spoonful or so of fluid out of the space which surrounds 

 the heart. It has the usual composition of lymph. It contains 

 proteins, but is not spontaneously coagulable. Leucocytes 

 are absent, a fact which probably accounts for its not clotting. 

 The fluid inside the cerebro-spinal system is extremely dilute. 

 Its principal salt its principal constituent, indeed is sodic 

 chloride. It contains hardly a trace of proteins, and these 

 in a modified condition proteoses. It also contains pyro- 

 catechin, a benzoic alcohol. This substance has long been 

 recognized as a constituent of cerebro-spinal fluid, owing to 

 the fact that, like sugar, it reduces copper salts when heated 

 with them in an alkaline solution. It appears to be one of the 

 products of proteid decomposition. Although exuded as lymph 

 from the bloodvessels of the chorioid plexuses, the composition 

 of cerebro-spinal fluid has been profoundly changed by the 

 activity it might almost be called the digestive activity 

 of the epithelium which lines the cerebro-spinal canal. There 

 is a theory that the ancestors of all vertebrate animals were 

 organized on a very different plan from that of their distant 

 descendants. Our cerebro-spinal canal was their stomach and 

 intestine. It would appear that the lining epithelium of these 

 organs, although disused for millions of years, cannot resist 

 the temptation to digest the lymph which they contain ! 

 The fluid in joints contains mucin (the essential constituent 

 of mucus), or a substance resembling mucin. In this case 

 the joint-membrane has added something to lymph without 

 removing or destroying any of its other constituents. 



Other illustrations might be given showing how the plasma 

 of blood is altered in composition while it is passing out of, 

 or after it has passed out of, capillary bloodvessels. Per- 

 haps it would be more logical to start on the outer side of the 

 walls of the capillaries ; since blood may, very properly, be 

 regarded as a tissue, dependent, like all other tissues, upon 

 diffusion from lymph for the nutrient materials that it needs. 



