ON THE BRAIN. 
519 
Kallomm, or hard medullary body; it is of firmer consistence 
than the medullary portion of the brain generally. I can plainly 
see the direction of the sides or supports of this arch, and by a 
little dissection I can clearly trace them. The anterior support 
or pillar I can follow down to between the corpora striata^ and 
the posterior ones are continuous with the fornix and the cornua 
ammonis. When I examine it more closely, I perceive that it 
is not only the roof of the ventricles, but a bond of union, or a 
connecting medium between the medullary matter of the supe¬ 
rior part of the brain on each side. I make a transverse incision 
horizontally across it, and continued over the ventricle on each 
side, and I can see the fibres or bands of medullary matter, run¬ 
ning from one hemisphere to the other: therefore it is that 
human anatomists call it the great commissure. Running along 
the centre of this body is a little depression or channel. It is 
occupied by one of the cerebral arteries, and is worthy of notice, 
as being the central line of the brain, and through which it 
might be divided longitudinally into two equal parts. 
Is of different Size in different Animals. —This corpus callo¬ 
sum, or great commissure, is, as you will perceive, very different 
in size in our different patients. It is, as you may observe, a body 
or band of very considerable bulk in the horse. In the ox it is 
considerably smaller, smaller even in proportion to the size of 
the two brains. In the dog it is larger than in the sheep ; it 
seems to bear a relative proportion to the development of the 
cerebral hemispheres. It increases in size in proportion to the 
rotundity of the brain and the intelligence of the animal, and is 
largest of all in man. It is developed in a direct ratio with the 
cerebellum in every animal, and also the tuber annulare and 
optic thalami, but it is in inverse proportion to the corpora 
quadrigemina and the spinal chord. 
- Not found in some animals. —The corpus callosum is said to 
be wanting in the hare, the rabbit, the rodentia generally, and in 
birds. There are no lateral ventricles in these animals of which 
it can become the roof, yet it exists in smaller size, as the com¬ 
missure or bond of union between the medullary matter in the 
superior portion of each hemisphere. The fibres or bands run¬ 
ning from one hemisphere to the other are more remarkable than 
where the corpus seems to have a double duty to perform. 
The Septum Lucidum. —Either dropping from the under side 
of the corpus callosum, or rising from the fornix, are two medul¬ 
lary laminae, partly united together by their interior surfaces, and 
called, from their semi-transparency, the septum lucidum. It is a 
medullary curtain let down between the lateral ventricles, and 
dividing them from each other, a communication between them 
