HEART AND BLOOD VESSELS 
451 
356. Clotting of Blood. — When the blood is exposed to the 
air it forms a clot. This 
is a peculiarity of blood 
alone. If it were not for 
this property of blood, 
animals would bleed to 
death from even a slight 
cut. In some warm 
blooded animals, the 
blood clots more quickly 
than in man. Man is 
able by pressing upon 
the blood vessels or by 
tying them to assist the 
process of clotting and 
prevent hemorrhage 
(hem'or-rag). Natu¬ 
rally the blood does not 
clot except when ex¬ 
posed to the air when 
fibrin threads are formed 
from the fibrinogen (fl- 
brin'o-jen) of the plasma 
of the blood. These 
threads hold the red and 
white corpuscles. After 
a short time the whole 
mass shrinks, squeezing 
out the fluid part of the 
blood, and the semi-fluid 
mass that remains is the 
clot. 
0 __ T _ , , , Figure 400 . — Organs of Circulation. 
357. Heart and Blood w . ,, , 
. Veins, black; arteries, with transverse lines. 
Vessels. I he blood IS L e ft s^g 0 f fig ure shows superficial vessels, 
carried from the heart while right side shows deeper vessels. 
