166 COMPARATIVE PHYSIOLOGY. 



action of all the various cells and tissues of the body. Any one 

 of these, departing from its normal behavior, at once affects the 

 blood ; but health implies a constant effort toward a certain 

 equilibrium, never actually reached but always being striven 

 after by the whole organism. The blood can no more maintain 

 its vital equilibrium, or exist as a living tissue out of its usual 

 environment, than any othef tissue. But the exact circum- 

 stances under which it may become disorganized, or die, ar.e 

 legion ; hence, it is not likely that the blood always clots in 

 the same way in all groups of animals, or even in the same 

 group. The normal disorganization or death of the tissue re- 

 sults in clotting ; but there may be death without clotting, as 

 when the blood is frozen, in various diseases, etc. 



To say that fibrin is formed during coagulation expresses in 

 a crude way a certain fact, or rather the resultant of many 

 facts. To explain ; When gunpowder and certain other ex- 

 plosives are decomposed, the result is the production of cer- 

 tain gases. If we knew these gases and their mode of com- 

 position but in the vaguest way, we should be in much the 

 same position as we are in regard to the cogulation of the 

 blood. 



There is no difficulty in understanding why the blood does 

 not clot in the vessels after death so long as they live, nor why 

 it does coagulate upon foreign bodies introduced into the blood- 

 stream. So long as it exists under the very conditions under 

 which it began its being, there is no reason why the blood 

 should become disorganized (clot). It would be marvelous if 

 it did clot, for then we could not understand how it could ever 

 have been developed as a tissue at all. It is just as reasonable 

 to ask, Why does not a muscle-cell become rigid (clot) in the 

 body during life? 



Probably in no field in physiology has so much work been 

 done with so little profit as in the one we are now discussing ; 

 and, as we venture to think, owing to a misconception of the real 

 nature of the problem. "We can understand the practical im- 

 portance of determining what circumstances favor coagulation 

 or retard it, both within the vessels and without them ; but 

 from a theoretical point of view the subject has been exalted 

 out of all proportion to its importance. 



Coagulation is favored by gentle movement, contact with 

 foreign bodies, a temperature of about 38° to 40° C, addi- 

 tion of a small quantity of water, free access of oxygen, etc. 



