THE BLOOD. 163 
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; and we should not have 
dwelt so long upon it, or burdened the student with some of 
the theories we have stated, except in deference to the views 
held by so many physiologists. 
It is not surprising that, looking at the subject with a dis- 
torted mental perspective, one theory should have replaced an- 
other with such rapidity. It is, however, of practical impor- 
tance to the medical student to remember some of the factors 
that hasten or retard, as the case may be, the coagulation of the 
blood. 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. 
The process is retarded by a low temperature, addition of 
abundance of neutral salts, extract of the mouth of the leech, 
peptone, much water, alkalies, and many other substances. 
The excess of carbonic anhydride and diminution of oxygen, 
seem to be the cause of the slower coagulation of venous blood, 
hence the blood long remains fluid in animals asphyxiated. A 
little reflection suffices to explain the action of most of the fac- 
tors enumerated. Any cause which hastens the disintegration 
of the blood-cells must accelerate coagulation; chemical changes 
underlie the changes in this as in all other cases of vital action. 
Slowing of the blood-stream to any appreciable extent likewise 
favors clotting, hence the explanation of the success of the 
treatment of aneurisms by pressure. It is plain that in all 
such cases the normal relations between the blood and the tis- 
sues are disturbed, and, when this reaches a certain point, death 
(coagulation) ensues, as with any other tissue. 
Clinical and Pathological_The changes in the blood that 
characterize certain abnormal states are highly instructive. If 
blood from an animal be injected into the veins of one of an- 
other species, the death of the latter often results, owing to non- 
adaptation to the blood already in the vessels, and to the tissues 
of the creature generally. The corpuscles break up—the change 
