98 
THE PHYSIOLOGY OF THE MEDICINAL LEECH. 
THE PHYSIOLOGY OF THE MEDICINAL LEECH.* 
BY JOHN B. HAYCRAFT, M.B., B.SC., F.R.S. (EDIN.), PROF. OF 
PHYSIOLOGY, MASON COLLEGE, BIRMINGHAM. 
When blood is withdrawn from the body it coagulates in 
a few minutes, and forms a solid mass. Not only is this seen 
in the cup into which the blood may have been shed, but 
occurs also in the region of the wound, filling this up, and 
plugging the little vessels which have been cut. But for 
this coagulation we should bleed to death from the slightest 
wound. Sometimes the blood loses its power to clot, and 
very serious consequences ensue. The study of coagulation 
is, therefore, very important, and physiologists and medical 
men have paid much attention to its investigation. 
While thinking over some obscure questions connected with 
this matter, in the autumn of 1888,1 recalled a fact familiar to 
every surgeon, viz., that after a leech bite the blood flows from 
the wound, and is very difficult to staunch ; and moreover 
that the blood which the leech has sucked remains permanently 
fluid within its stomach. So much was known at that time 
about the coagulation that I was able to predict an explanation 
of these curious and hitherto unexplained facts, which my 
experiments enabled me to confirm. These I will now 
describe, confining myself to those points which will be of 
most interest to the general biologist. 
A blood-clot consists of a sponge of albumen (fibrin) which 
encloses the corpuscles in its meshes. When shed from the 
body the blood contains red and white microscopic particles— 
corpuscles—floating in a fluid—the liquor sanguinis. Very 
soon the white corpuscles form small quantities of a very 
active substance, a kind of ferment, which acts on the liquor 
sanguinis, and causes the formation in it of this sponge of 
fibrin. Now this is prevented in some way by the leech. 
If you cut out with a pair of sharp-pointed scissors the 
sucker and gullet of a leech, and after chopping these into 
very small pieces, place them for an hour or so in very weak 
salt solution, you will obtain a watery solution, a specimen of 
which I show you in this bottle. If I add a drop of this to 
a few drops of blood freshly drawn it will remain quite fluid, 
while this similar portion will clot in a few minutes. 
* Transactions of the Birmingham Natural History and Micro¬ 
scopical Society. Abstract of an Address, read December 9th, 1884. 
