THE CEREBRUM OF VERTEBRATA. 325 



hemispheres, the number of its secondary convolutions, and the absence of 

 symmetry in the forms and dispositions of all the convolutions, constitute the 

 three great external distinguishing characters of the human brain. 



If, finally, we regard the general plans, and the secondary arrangements, of 

 the cerebral hemispheres in the different Vertebrata, from a physiological 

 point of view, we find a close general correspondence between the amount of 

 intelligence manifested by the several members of this series, and the degree 

 of complexity of the cerebral hemispheres. It has even been noticed that, in 

 different varieties of one kind of animal, subjected to different conditions, as 

 regards education, and therefore exhibiting various grades of active intelli- 

 gence, the general development and size of the cerebrum, keep pace with the 

 growth of that intelligence ; for the brains of trained and domesticated dogs, 

 are proportionally larger than those of the wild dog. But hitherto, the at- 

 tempts made by psychologists and comparative anatomists, to associate, with 

 certainty, particular parts of these hemispheres, with particular psychical 

 endowments, have proved abortive. 



The yanglionic masses at the base of the cerebrum. The corpora quadrigemina 

 are still divided, as their name implies, into four eminences throughout all the 

 Mammalia ; the anterior pair are larger in the Herbivora, and the posterior 

 pair in the Carnivora. In Birds, Reptiles, and Fishes, these bodies are bi- 

 geminal, consisting of only a single pair of tubercles, or ganglionic masses, 

 which are closely attached to the optic thalami, and form the so-called optic 

 lobes; in the osseous fishes, these optic lobes include the optic thalami, or sup- 

 posed centre of common sensation. The corpora striata are always, through- 

 out the whole of the Yertebrata, as in man, concealed by the cerebral hemi- 

 spheres ; but as these latter parts are more and more reduced in size, the striated 

 bodies become more and more apparent, and at last, in the Fishes, constitute 

 almost alone, the so-called cerebral lobes, a very thin layer of gray matter, 

 forming the only remnant, or representative, of the cerebral hemispheres. In 

 Birds, and in the higher cartilaginous fishes, a ventricular cavity, into which 

 the striated bodies project, is found in these cerebral lobes. It has been sug- 

 gested, that not only consciousness of simple sensations, but their perception, 

 or the reference of these to their proper external objects, is accomplished in 

 the sensorial ganglia at the base of the cerebrum, at least in these lower Yer- 

 tebrata (Carpenter) ; but this view is hypothetical. The size of the optic, and 

 also that of the olfactory lobes, varies in different groups of animals ; thus the 

 olfactory lobes are larger in animals which possess an acute sense of smell, as 

 in certain Carnivora, in the Ruminantia and Rodentia, and even in the shark 

 tribe amongst Fishes ; whilst the optic lobes are very large in Birds, animals 

 in which the sight is very powerful, and also in certain Fishes. 



Structure of the Cerebrum. The internal structure of the cerebral hemi- 

 spheres, also undergoes simplification in the descending vertebrate series. As 

 we have seen, the lateral ventricles become smaller in extent, and of simpler 

 form, their posterior cornua being absent, except in the seal, in animals lower 

 than the Quadrumana. The thickness of the gray matter of the hemispheres 

 also gradually diminishes, in passing from the higher to the lower Yertebrata. 

 The layer of cortical substance in Fishes is so thin, that these parts of the 

 encephalon appear almost white to the naked eye. It is obvious, that the 

 numerous layers distinguishable in the cortical substance in man, and which 

 have also been seen in the Mammalia, must gradually become fewer, and at 

 last disappear. The quantity of medullary substance, although proportionately 

 to the gray matter greater, is also absolutely diminished, and the arrangement 

 of its fibres becomes much simplified. This appears especially to be the case 

 as regards the fibres which, in the higher brains, pass from one set of con- 

 volutions to another set, or, in the smooth brain, from one part of a hemi- 

 sphere to the other. So, too, a very significant diminution takes place in 

 the number of the transverse commissural fibres which serve to unite the right 

 and left halves of the cerebrum, and to bring their respective bilateral parts 

 into physiological connection. The corpus callosum, for example, which, in 

 the brain of man, and in that of the higher Mammalia, is of such great ex- 

 tent and thickness, and contains the chief part of these transverse commissural 

 fibres, has been shown to be relatively smaller, even in the highest anthropoid 



