THE PHYSIOLOGY OF THE MEDICINAL LEECH. 
99 
We have extracted from the leech then a substance which 
prevents coagulation. What is it ? I am not in a position 
to answer this question. I have tried to find out, and one of 
the most distinguished of the German chemists, Professor 
Schmiedeberg, has tried also, but without success. The 
smallest quantity only is present, but it has an action on the 
blood as powerful in its way as the venom of the rattlesnake. 
A quantity of the substance, obtained as yet in an impure 
state, less than a grain in weight, will prevent a gallon of 
blood from clotting. 
This substance the leech secretes from its sucker; and if 
this organ be examined with a microscope, a large number of 
little glands will be seen opening on its surface. These are 
single cells and they may be compared with the salivary 
glands of man, and their secretion—containing the substance 
—with the saliva. 
The saliva of the leech prevents coagulation. How does 
it operate ? It kills the ferment which produces the fibrin 
from the liquor sanguinis. The experiments conducted in 
order to prove this point would take long to describe, but I 
may mention that the saliva although it kills the ferment does 
not kill the cells which produce it. If a drop of blood be 
mixed with a drop of this extract of leech saliva and examined 
with the microscope, carefully warming the preparation with 
suitable apparatus, the little white corpuscles will be seen 
moving about as in normal blood. 
In this preparation you will see the blood of a crab under 
the microscope. There are a mass of white corpuscles—no red 
ones exist—welded together by processes of their protoplasm 
called pseudopodia. This forms the clot seen in invertebrate 
blood, which is then due not to the formation of fibrin, but 
to the fusion of the white corpuscles. We have seen that 
in human blood the leech saliva does not affect the white 
corpuscles. These are homologous of those of the crab just 
alluded "to, and we should anticipate then that the saliva will 
not prevent the clotting of crab’s blood. This is the case. 
If a small quantity of the extract be injected into the jugular 
vein of a rabbit or dog the animal will be thrown into a very 
curious condition, in which it resembles a patient suffering 
from a disease called haemophilia. The slightest wound in 
the skin continues to bleed. In haemophilia this may lead to 
fatal consequences, but as the leech saliva is eliminated pretty 
rapidly from the system, its injection is not so very serious a 
matter. 
Now you will be in a position to see the reason why 
the blood continues to flow for so long a time from the leech- 
