320 THE AMERICAN NATURALIST.  [VOL. XXXIIL 
and employed in the brain with such a resulting effect upon our 
body musculature that the desired object, the preservation of 
equilibrium, is attained. 
We have now to investigate further what part of the brain is 
intrusted with this important and complicated function. 
First, I may again point out that we are, as a rule, entirely 
unconscious of many of the sensations concerned in this process. 
And since we are justified in concluding that, in order to be- 
come conscious, sensations must pass to the cortex of the cere- 
brum, we are from the first inclined to look for the organ of 
equilibration which controls the codrdination of the muscles 
elsewhere than in the cortex of the cerebrum. There is also 
a series of facts which makes it highly probable that the part of 
the central apparatus sought is to be found in the cerebellum. 
The cerebellum is an organ which is clearly separated from 
the rest of the central nervous system, and which in its finer 
structure differs essentially from all other parts of the brain. 
It has also a special and peculiar function. In comparison with 
the other parts of the brain it is not proportionately well devel- 
oped in all animals. In mammals and birds the cerebellum is 
not only relatively large, but its surface is increased many times 
by numerous delicate and usually deep parallel furrows. In the 
Amphibia and many reptiles, on the contrary, it is reduced to 
a simple small ridge. While the first-mentioned groups require 
very sensitive and complicated muscular action for the mainte- 
nance of the equilibrium while standing, running, and flying, 
in the other groups a much simpler apparatus suffices for mere 
crawling or jumping. In the fishes the cerebellum is larger 
than in the Amphibia, yet with the exception of one of the 
cartilaginous fishes it is still smooth. Frogs, it is true, swim, 
but with far less rapidity and precision than fish. Thus we 
actually find a certain parallelism between the size of the cere- 
bellum and the delicacy of the muscular codrdination. 
Another anatomical fact is now to be considered. We are 
acquainted with certain tracts of the spinal cord which we know 
convey to the brain a part of the sensations necessary for the 
preservation of the equilibrium. We know that, directly or 
indirectly, all of these tracts are intimately connected with the 
