252 MANUAL OF PHYSIOLOGY. 



made up of living constituents requiring constant assimilation 

 and elimination for the maintenance of its perfectly normal condi- 

 tions and life. One can confidently say that coagulation is the 

 outcome of certain chemical changes concomitant with the death 

 of this tissue, and that while the tissue lives no such changes 

 take place. But such an answer adds little to our knowledge of 

 the matter. 



Since constant chemical intercourse must be kept up between 

 the blood and its surroundings in order to sustain the complex 

 chemical integrity essential for its life, we cannot be surprised 

 that its waste materials accumulate, and that it soon dies when 

 shed, just as other tissues do when deprived of their means of 

 support. The formation of a solid and the separation of a liquid 

 form of proteid is in no way unusual as a first step in the decline 

 from exalted chemical construction, for similar changes occur in 

 other tissues, and in protoplasm itself. The soft, contractile sub- 

 stance of muscle, probably during its contraction, and certainly 

 at its death, tends to undergo almost exactly the same kind of 

 change as the blood in coagulation. 



If we knew accurately the nutritive process taking place in 

 the blood itself, and with which of its surroundings it keeps up 

 chemical interchange, the answer would be much simplified. But 

 we have in the blood three elements that probably have different 

 modes of assimilation and elimination, viz., plasma, white cells, 

 and red disks. But we practically know nothing of the changes 

 they undergo during their nutrition ; or whether their tissue 

 changes have any necessary relation to those of the neighboring 

 tissues. We do know, however, that there exists some very inti- 

 mate relation between the membrane lining the vessel walls and 

 the contained blood. They seem to require frequently- repeated 

 contact one with the other in order that the normal condition of 

 both may be maintained in perfect, vital integrity. Thus fresh 

 supplies of blood are required by the vessel wall, for, when de- 

 prived of its nutriment by a stoppage of the blood flow, it soon 

 loses its power of retaining the blood, and admits of extravasa- 

 tion ; and renewed contact with the vessel wall is equally neces- 

 sary for the blood, for the cells congregate, and the plasma, when 



