70 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 loAver 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 Kodent 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 



