SECTION V 

 THE LYMPH 



CHAPTER XXI 

 PROPERTIES AND FORMATION OF LYMPH 



General Consideration. The lymph forms a medium of inter- 

 change between the blood and the tissues. This is made necessary by 

 the fact that the blood does not come in actual contact with the cells, 

 but remains separated from them by the lining of the capillaries. It 

 thus plays the part of a middleman, and carries nutritive material 

 to the cells in exchange for the products of their metabolism. It is 

 true, however, that the importance of the lymph as a distributing 

 agent varies in different tissues, because some of them are more vascular 

 than others, and are equipped for this reason with a more intricate net- 

 work of blood-capillaries. The individual cells are thus brought into 

 closer relation with the blood-stream. Under less favorable conditions 

 relatively large numbers of cells are grouped around a single blood 

 channel, so that the nutrition of the outlying elements can only be 

 effected by a correspondingly greater development of the lymphatic 

 vessels and spaces. In fact, some tissues are free from blood-vessels, 

 their nutrition being carried on by the lymph filling the delicate inter- 

 cellular spaces permeating them. An arrangement of this kind is 

 present in the central zone of the cornea through which the rays of 

 light enter the eye. It need scarcely be mentioned that the presence 

 of blood-capillaries in this particular structure would tend to hinder 

 the refraction of the light rays. 



The term lymph is generally applied to that part of the body fluid 

 which is contained in the preformed lymphatic channels, while that 

 part of it which bathes the individual cells, is designated as tissue 

 fluid. This classification has some points in its favor, because the 

 intercellular spaces are not always directly continuous with the larger 

 central channels, but are at times separated from them by delicate 

 membranous partitions. As the latter are only semipermeable, it 

 usually happens that the composition of the tissue-fluid is slightly 

 different from that of the intravascular lymph. Lymph, however, 

 originates in all parts of the body and all types of lymphatic fluids 

 contribute toward its formation. For this reason, it seems desirable 

 to include under this heading also those liquids which are contained 

 in the different serous spaces of the body, for example, in the peri- 

 cardial, pleural and peritoneal cavities, and in the spaces of the 



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