160 COMPAKATIVE PHYSIOLOGY. 



the spleen, liver, lymphatic glands, and other tissues of similar 

 structure contribute in a less degree to the development of the 

 red corpuscles ; and that the last mentioned organs are the chief 

 producers of the colorless amoeboid blood-cells. 



Finally, it is well to remember that Nature's resources in 

 this, as in many other cases, are numerous, and that her mode 

 of procedure is not invariable ; and that, if one road to an end 

 is blocked, another is taken. 



The SecUne and Death of the Blood-Cells.— The blood cor- 

 puscles, like other cells, have a limited duration, with the usual 

 chapters in a biological history of rise, maturity, and decay. 

 There is reason to believe that the red cells do not live longer 

 than a few weeks at most. The red cells, in various degrees of 

 disorganization, have been seen within the white cells (phagocy- 

 tes), and the related cells of the spleen, liver, bone-maxrow, etc. 

 In fact, these cells, by virtue of retained ancestral (amoeboid) 

 qualities, have devoured the weakened, dying red cells. It seems 

 to be a case of survival of the fittest. It is further known that 

 abundance of pigment containing iron is found in both spleen 

 and liver ; and there seems to be no good reason for doubting 

 that the various pigments of the secretions of the body (urine, 

 bile, etc.) are derived from the universal pigment of the blood. 

 These coloring matters, then, are to be regarded as the excreta 

 in the first instance of cells behaving like amoeboids, and later 

 as the elaborations of certain others in the kidney and else- 

 where, the special function of which is to get rid of waste prod- 

 ucts. The birth-rate and the death-rate of the blood-cells must 

 be in close relation to each other in health ; and some of the 

 gravest disturbances arise from decided changes in the normal 

 proportions of the cells (ancemia, leucocythemia). 



Both the red and white corpuscles show, like all other cells 

 of the organism, alterations corresponding to changes in the 

 surrounding conditions. The blood may be withdrawn and its 

 cells more readily observed than those of most tissues ; so that 

 the study of the influence of temperature, feeding of the leuco- 

 cytes, and the action of reagents in both classes of cells is both 

 of practical importance and theoretic interest, and will well re- 

 pay the student for the outlay in time and labor, if attention is 

 directed chiefly to the results and the lessons they convey, and 

 not, as too commonly happens, principally to the methods of 

 manipulation. 



The Chemical Composition of the Blood.— Blood has a decided 



