7O GENERAL ANATOMICAL CHARACTERS 
spheres have a greater preponderance compared with other parts, 
especially to the so-called optic lobes, or corpora quadrigemina, 
which are completely concealed by them. The commissural system 
of the hemispheres is much more complex, both fornix and corpus 
callosum being present in some form; and when the latter is 
rudimentary, as in Marsupials and Monotremes, its deficiency is 
made up for by the great size of the anterior commissure, The 
lateral lobes of the cerebellum, wanting in lower vertebrates, are 
well developed and connected by a transverse commissure, the pons 
Varolii. The whole brain, owing especially to the size of the 
cerebral hemispheres, is considerably larger relatively to the bulk 
of the animal than in other classes, but it must be recollected that 
the size of its brain depends upon many circumstances besides the 
degree of intelligence which an animal possesses, although this is 
certainly one. Man’s brain is many times larger than that of all 
other known mammals of equal bulk, and even three times as large 
as that of the most nearly allied Ape. Equal bulk of body is here 
mentioned, because, in drawing any conclusions from the size of 
the brain compared with that of the entire animal, it is always 
necessary to take into consideration the fact that in every natural 
group of closely allied animals the larger species have much smaller 
brains relatively to their general size than the smaller species, so 
that, in making any effective comparison among animals belonging 
to different groups, species of the same'size must be selected. It 
may be true that the brain of a Mouse is, as compared with the 
size of its body, larger than that of a Man, but, if it were possible 
to reduce an animal having the general organisation of a Man to the 
size of a Mouse, its brain would doubtless be very many times larger ; 
and conversely, as shown by the rapid diminution of the relative 
size of the brain in all the large members of the Rodent order, a 
Mouse magnified to the size of a Man would, if the general rule 
were observed, have a brain exceedingly inferior in volume. Al- 
though the brain of the large species of Whales is, as commonly 
stated, the smallest in proportion to the bulk of the animal of any 
mammal, this does not invalidate the general proposition that the 
Cetacea have very large brains compared with terrestrial mammals, 
like the Ungulata, or even the aquatic Sirenia, as may be proved 
by placing the brain of a Dolphin by the side of that of a Sheep, a 
Pig, or a Manatee of equal general weight. It is only because the 
universally observed difference between the slower ratio of increase 
of the brain compared with that of the body becomes so enormous 
in these immense creatures that they are accredited with small 
brains. 
The presence or absence of “sulci” or fissures on the surface 
of the hemisphere, dividing it into “convolutions” or “ gyri,” and 
thus increasing the superficies of the cortical gray matter, as well 
