ON THE BRAIN. 
525 
uppermost are the nates, the lower ones the testes. The trans¬ 
verse line is differently placed in different animals. In herbivo¬ 
rous animals it is placed lower, so that the nates are most deve¬ 
loped ; in carnivorous animals, it is found higher, so that the 
testes preponderate, and in omnivorous animals, as in the human 
being, the transverse line is nearly in the centre, and there is 
little difference in the size of the tubercles. The observation of 
these brains of the sheep, the dog, and the swine, will impress 
this on your minds. You need only to see the tubercula quadri- 
gemina in order to decide on the character of the animal as de¬ 
signated by his food. 
Proportion to other Parts of the Brain .—These tubercles pre¬ 
sent us with another lesson of comparative anatomy. Observe 
how much larger they are, compared with the bulk of the brain, 
in the ox than in the horse, and particularly how much more sc 
than in the dog. Their development is in proportion to that of 
the optic nerve and the spinal chord, and in an inverse propor¬ 
tion to that of the tuber annulare. Connected with this, it is 
evident that, in proportion to the bulk of these tubercles, that of 
the cerebral hemispheres is diminished, or the animal principle 
preponderates over the intellectual. We have already seen, that 
the optic thalami bear also an inverse proportion to the tubercles. 
On these points of coincidence and of difference, we shall pro¬ 
bably be hereafter enabled to found a rational theory of the func¬ 
tions of the various parts of the brain. In the hare and rabbit, 
and the rodentia generally, these tubercles have an extraordinary 
development. In these animals there are no lateral ventricle, and 
the quadrigemina are hollowed out to form lesser ventricles, that 
probably answer the purpose of the lateral ones in others. An 
incision into these tubercles will bring to view numerous faint 
strige of cineritious and medullary matter. At the base of the 
testes is a medullary band or tract, which reaches as far as the 
optic thalami, and connects these prominences together ; and 
also a medullary curtain or screen, somewhat resembling the sep¬ 
tum lucidum, rises up, connecting itself with the crura cerebelii, 
and forming a kind of roof for the anterior part of the fourth 
ventricle. 
The Crura Cerebri .—I now turn to the base of the brain, and 
there I observe in every domesticated quadruped the great pre¬ 
ponderance of medullary matter. I can distinctly trace three 
lobes on each side ; on the two anterior or inferior ones, I recog¬ 
nize the mammillary processes resting upon the aethmoid bone, 
and abutting upon the cribriform plate of the same bone, and 
also the distinct and largely-developed bulk of the olfactory 
VOL. VI. 3 Y 
