CHAPTEK VII. 



THE BLOOD AND THE LYMPH. 



WE have discussed already the ways in which the various nutrients 

 (including oxygen) become incorporated in the organism to furnish it 

 energy and the materials for replacing its ever-wasting tissues. The 

 means by which the oxygen and the absorbed food products are dis- 

 tributed to the myriad tissue-cells, some very remote, and by which the 

 waste of these cells is removed, remain to be described. These means 

 are, of course, the blood and lymph. Combined, these constitute one of 

 the most essential of bodily tissues. Blood requires two modes of 

 description, one of its chemical composition and the other of its morpho- 

 logical structure. 



Physically human blood consists of a liquid bearing within it two sorts 

 of corpuscles and little masses known as platelets not properly classed 

 as corpuscles. The liquid is called plasma and the three kinds of bodies 

 are the erythrocytes, or red corpuscles, the leukocytes, or white corpuscles, 

 and the thrombocytes, or platelets. In chemical composition blood is 

 exceedingly complex, theoretically more so even than the varied tissues, 

 for it contains not only the materials on which the tissues live, but also the 

 multifarious end-products of tissue-katabolism. Lymph is so nearly 

 like blood-plasma in its composition that they are properly studied 

 together. 



The Chemical Composition of the Blood and Lymph has been more or less 

 described, indirectly, in the preceding chapters, but requires to be set 

 forth systematically. We shall then all the better appreciate in how 

 literal a sense the circulation is but a means of distributing the food, 

 oxygen, and heat, and of collecting the waste, of the body. Because these 

 two processes comprise one of the largest functions of life, nutrition, a 

 good knowledge of the blood is of unexcelled importance, for this com- 

 plex "liquid" is indeed "life-blood." When it stops moving in its devious 

 way among the cells life departs and death more or less gradually comes 

 on. Mammalian blood, as we have seen, consists of plasma containing 

 erythrocytes, leukocytes, and thrombocytes. Each of these, as well as 

 lymph and serum, has functions and in consequence a chemical composi- 

 tion peculiar to itself which must be outlined each in its turn. The 

 functions and the physical composition of the whole blood will be then 

 the better understood. 



The following substances appear to be regular constituents of human 

 blood in its normal circulating condition. There are doubtless many 

 others in mere traces and perhaps still others of importance not yet 



