ed and in the case of
some animals altogether prevented. For example, human blood kept at body
temperature clots in three minutes, while if allowed to cool to room
temperature the first sign of clotting may not make its appearance until
eight minutes after its removal from the body. The process of clotting
is also considerably accelerated by making the blood flow in a thin
stream over a wide surface. The full completion of the process occupies
some time if the blood be kept quiet, but ultimately the whole mass of
the blood becomes converted into a solid. At this stage the containing
vessel may be inverted without any drop of fluid escaping. A short time
after this stage has been reached drops of a yellow fluid appear upon
the surface and, increasing in size and number, run together to form a
layer of fluid separated from the clot. This fluid is termed _serum_;
its appearance is due to the contraction of the clot, which thus
squeezes out the fluid from between its solid constituents. Contraction
continues for about twenty-four hours, at the end of which time a large
quantity (one-third or more of the total volume) of serum may have been
separated. The clot contracts uniformly, thus preserving throughout the
same general shape as that of the vessel in which the blood has been
collected. Finally the clot swims freely in the serum which it has
expressed.

The cause of the clot formation has been found to be the precipitation
of a solid from the liquid plasma of the blood. This solid is in the
form of very minute threads and hence is termed _fibrin_. The threads
traverse the mass of blood in every possible direction, interlacing and
thus confining in their meshes all the solid elements of the blood. Soon
after their deposition they begin to contract, and as the meshwork they
form is very minute they carry with them all the corpuscles of the
blood. These with the fibrin form the shrunken clot.

If the rate at which blood clots be retarded either by cooling or by
some other process the corpuscles may have time to settle, partially or
completely, in which case distinct layers may form. The lowermost of
these contains chiefly the red corpuscles, the second layer may be grey
owing to the high percentage of leucocytes present, while a third,
marked by opalescence only, may be very rich in platelets. Above these a
clear layer of fluid may be found. This is _plasma_. The formation of
these layers depends solely upon the rate of sed