THE BLOOD. 33 



from the disintegration of tissues and return them in 

 a modified form to the blood. 



50. The Blood. — The blood is the fluid from which 

 the material constituting all the tissues of the l)ody 

 is derived. It exists at a very early period of develop- 

 ment, before the tissues exhibit their structural 

 character or special properties. The food we take is 

 not directly converted into tissue, but into blood. 

 Neither are the substances resulting from the breaking 

 down of tissues at once converted into the materials 

 which are excreted from the body by the lungs, liver, 

 kidneys, skin, and intestines, but into blood. The 

 order of change therefore may be represented thus 

 1. food; 2. Hood; 3. tissue; 4. products of decay ; 

 5. hlood ; 6. excretedj maiter. But if we take more 

 food than our body requires, the excess, after having 

 been converted into blood, is excreted without being 

 first converted into tissue. This excess is not only 

 useless, but its excretion may overtax our organs and 

 damage them very seriously, particularly if they are 

 not of large size and in thoroughly good order. Ex- 

 cess of food is not unfrequently in this way the cause 

 of derangement of certain important organs, particu- 

 larly the liver and kidneys, and may occasion early 

 death. The blood is being continually renovated bv 

 the conversion of food into it. The food does not 

 pass directly into the blood-vessels, but certain of its 

 constituents are selected and absorbed by the living 

 matter of the walls of the blood-vessels, and by that 

 of the blood itself. In this way the food is changed 

 and converted before it becomes blood. 



51. Bioplasm of the Blood.— The blood is not a 

 mere solution of food matter, but contains many 

 living particles in suspension, which circulate with 

 it, and grow and multiply, and arrive at matu- 

 rity and die, becoming resolved into matters which 

 are dissolved in the liqaid which permeates the walls 

 of the vessels, and is applied to the nutrition of the 



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