116 THE BLOOD AND THE LYMPH 



are taken up by special large cells showing phagocytic powers, and trans- 

 ferred to the lymphatics — for example, those of the diaphragm. A sim- 

 ilar selective absorption is well known in the ease of the villi of the in- 

 testine, where fat passes into the lacteals and carbohydrates into the 

 blood. It appears as if lymphatic adsorption, both of solid materials 

 and of solutions, requires the cooperation of phagocytic cells. 



The newer conception of the lymphatics as a closed system is at vari- 

 ance with the older one, in which they were supposed to get smaller and 

 smaller, and their walls less and less complete until ultimately they 

 faded off into the tissue spaces. These, however, bear no closer relation- 

 ship to lymphatics than they do to blood capillaries. The tissue spaces 

 include all the minute spaces between the fibers and cells of the con- 

 nective tissues and between the parenchyma of the organs and the great 

 serous cavities of the body (pleural, peritoneal), as well as specially 

 developed tissue spaces, forming the subarachnoid spaces of the brain, 

 the scala vestibuli and tympani of the cochlea and the anterior chamber 

 of the eye. The fluids in these spaces — the tissue fluids — are quite dif- 

 ferent from the lymph in the lymphatics both in composition and in 

 function. Indeed, the tissue fluids are among the most varied of all 

 the fluids of the body. The spaces may themselves become linked to- 

 gether so as to form a circulatory system, which is quite independent of 

 the lymphatics. This is particularly the case in the brain, where the tis- 

 sue spaces surrounding every individual nerve cell extend into the sub- 

 arachnoid area, where they drain into the cerebral sinuses through the 

 arachnoidal villi, which exist as lace-like projections of the arachnoid 

 into the dural sinuses, being covered by a layer of mesothelial cells spe- 

 cially abundant at the tips of the villi, where they form cell nests. Ob- 

 servations of the passage of substances in solution by these pathways 

 have been made by injecting potassium ferrocyanide and citrate of iron 

 into the subarachnoid and subdural spaces and afterwards detecting 

 the presence of the salts by mounting sections in acid media, so as to 

 permit prussian blue to develop. Ordinarily the precipitate is found in 

 or near the villi, but after cerebral anemia it forms in the tissue spaces 

 that surround the nerve cells. 



There are therefore three fluids concerned in the transference of food 

 materials and gases between the gastrointestinal apparatus and lungs 

 and the tissue cells — namely, the blood plasma, the tissue fluids, and the 

 lymph. The tissue fluid, being in contact with the tissue elements, serves 

 as their immediate nutritive fluid, and it is the function of the blood and 

 lymph to maintain it of proper composition. Everything must be trans- 

 ferred to and from the tissue cells through the tissue fluid, making it 



