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682 GENERAL FA THOL G Y.

a great result by causing changes in other substances without itself
entering into the combination.
Thrombi formed in the manner just stated are always white or nearly
so. This is always the case when they are formed slowly in the blood-
stream, from the fact that very few red blood-globules or none become
entangled in the forming mass. This circumstance enables the pathol-
ogist to determine whether or not a given blood-clot has been formed
Fig. 375.

Fig. 374.

A Thrombus, ten days old, after
moditied ligation. Longitudinal
cut. Low power. Afier ligature
at A, the artery was seized and
compressed at B between the
arms of a pair of forceps, a, ad-
ventitia ; r», media ; c, cellular
tissue p, cellular formation at
;
bottom of clot, non-organized,
and apparently not larger than
such an accumulation usually is
at five days ; it consists mainly of
cells similar to white blood-cor-
puscles; only a few epithelioid
cells are .scattered through it,
and no granulations springing
from it penetnite the crevices
of the laminated clot (d) imme-
Longitudinal Section of the I.igatiired Kiid diately above ; at p', ;»" there is
of the (rural Artery of a Dog, fifty days an ingrowth of the intima and
after the application of the ligature, show- inner layers of the media. At L,
ing the newly-formed vessels in the throm- above the point of compression,
bus and their communications with the a blood-clot like that at il rested,
vasa vasorum : 7//, thrombus; J/, muscular but handling caused its disjjlace-
coat ; Z, external coat (X 20, O. Weber). ment.
])v gradual growth on account of some injury, inflammatory or other-
wise, to the vessels, or from stagnation of the blood. This character of
clot can be closely imitated artificially by rapidly whipj^ing freshly-
drawn blood with a bundle of small twigs, to which the forming fibrin
will adhere, leaving the red globules in the blood-scrum. By working
quickly and carefully the clot adhering to the twigs will be nearly
white, and all the fibrin can in this way be taken from the blood. This
is the mode of preparing defibrinated blood ior transfusion.
But one cause is known for the formation of thrombi in the living
vessels, and this is some actual injury to their walls. This injury may
be effected in a multitude of ways. On account of pressure or extreme
weakness of the heart the blood may stagnate in the vessels luitil the
endothelium becomes seriously impaired. Arteritis may affect them, or,