158 ANIMAL PHYSIOLOGY. 
sels it begins to thicken, and gradually acquires a consistence 
that may be compared to that of jelly, so that it can no longer 
be poured from the containing vessel. Though some have rec- 
ognized different stages as distinct, and named them, we think 
that an unprejudiced observer might fail to see.that there 
were any well-marked appearances occurring invariably at the 
same moment, or with resting stages in the process, as with 
the development of ova. 
After coagulation has reduced the blood to a condition in 
which it is no longer diffluent, minute drops of a thin fluid 
gradually show themselves, exuding from the main mass, 
faintly colored, but never red, if the vessel in which the clot 
has formed has been kept quiet so that the red corpuscles have 
not been disturbed; and later it may be noticed that the main 
mass is beginning to sink in the center (cupping); and in the 
blood of certain animals, as the horse, which clots slowly, the 
upper part of the coagulum (crassamentum) appears of a 
lighter color, owing, as microscopic examination shows, to the 
relative fewness of red corpuscles. This is the buffy-coat, or, as 
it occurs in inflammatory conditions of the blood, was termed 
by older writers, the crusta phlogistica. Itis to be distinguished 
from the lighter red of certain parts of a clot, often the result 
of greater exposure to the air and more complete oxidation in 
consequence. The white blood-cells, being lighter than the red, 
are also more abundant in the upper part of the clot (buffy- 
coat). If the coagulation of a drop of blood withdrawn from 
one’s own finger be watched under the microscope, the red cor- 
puscles may be seen to run into heaps, like rows of coins lying 
against each other (rowleaux, Fig. 145), and threads of the 
greatest fineness are observed to radiate throughout the mass, 
gradually increasing in number, and, at last, including the 
. whole in a meshwork which slowly contracts. It is the forma- 
tion of this fibrin which is the essential factor in clotting; the 
inclusion of the blood-cells and the extrusion of the serum 
naturally resulting from its formation and contraction. 
The great mass of every clot consists, however, of corpus- 
cles; the quantity of fibrin, though variable, not amounting to 
more usually than about ‘2 per cent in mammals. The forma- 
tion of the clot does not occupy more than a few minutes (two 
to seven) in most mammals, including man, but its contraction 
lasts a very considerable time, so that serum may continue to 
exude from the clot for hours. It is thus seen that, instead of 
the plasma and corpuscles of the blood as it exists within the 
