THE CEREBRUM OF VERTEBRATA. 323 



end of the Sylvian fissure, one above the other, and pass continuously from 

 the anterior or frontal, into the middle or parieto-temporal lobe. In neither 

 of these Orders, nor in those lower in the scale, as the Rodents, Marsupials, 

 and Monotremes, is there a distinct portion of the hemispheres, marked off 

 from the other lobes, to form a posterior or occipital lobe ; indeed, in these 

 latter groups, even the middle lobe seems to be rudimentary. In the Rumi- 

 nants and Pachyderms, traces of a fissure of Rolando may be detected ; but in 

 none of the preceding groups, not even in the Carnivora, excepting in the seal, 

 is there to be found, within the cerebrum, a prolongation backwards of the 

 lateral ventricle, in the form of a posterior cornu ; in the seal, a rudimentary 

 cornu, with its contents, first appears. It is not to be inferred from this, 

 however, that there does not exist some small portion of the cerebral hemi- 

 spheres of these animals, which is anatomically and physiologically homolo- 

 gous with, or representative of, the parts called the posterior lobes in the still 

 higher Mammalia, and in Man ; but all w r e are entitled to say is, that the plan 

 of structure of the hemispheres, excludes such subdivisions or markings, as 

 serve to distinguish a part, as a posterior or occipital lobe. In all the so- 

 called Quadrumana, however, which amongst the Mammalia are nearest to 

 man, and which, as we have seen (p. 107) have been zoologically associated 

 with him, in a common Order, named Primates, another plan prevails, in the 

 arrangement of the substance of the cerebral hemispheres, which indeed corre- 

 sponds fundamentally, with the plan observed in the human cerebrum. It is 

 one part of this plan, that the existence of a posterior or occipital lobe, should 

 be indicated internally, by the extension backwards, into that part of the 

 hemisphere, of an included prolongation of the lateral ventricle, forming a dis- 

 tinct posterior cornu ; besides this, when traces of fissures appear upon the 

 surface of the hemispheres, they occupy the position of the principal fissures 

 of the human brain, viz., the fissure of Rolando, marking off the frontal from 

 the parietal lobe ; the internal perpendicular fissure, distinguishing, even on 

 the surface, the occipital, or posterior, from the parietal, or middle, lobe ; and 

 the fissure of the hippocampi, formed by the folding inwards of the cerebral 

 substance, along the floor of the posterior cornu. In the higher monkeys, the 

 baboons, and the anthropoid apes, other sulci appear between the principal 

 fissures, serving, step by step, to complicate the cerebral surface, and to mark 

 it off, into more and more numerous convolutions, the general arrangement of 

 which is undoubtedly correspondent with that traceable in the far more com- 

 plex array of convolutions in the human brain. In all these, there may be 

 recognized certain primary frontal, parietal, occipital, and temporal convolu- 

 tions, which have, as in the Ruminant and Carnivorous brains, a general 

 longitudinal direction ; and, in the higher forms, secondary convolutions are 

 interposed. Both the primary and secondary convolutions become progres- 

 sively more tortuous ; and certain of the latter are met with only in man. It 

 is beyond doubt, however, that the cerebrum of the so-called Quadrumana, and 

 that of Man, are constructed on a common plan ; but when we consider its 

 absolute size, or, more especially, its relative size, as compared with the cere- 

 bellum, the spinal cord, or the entire body, the relative development in particu- 

 lar of its frontal and parietal regions, furthermore, the number and complexity 

 of its secondary convolutions, and lastly, the thickness of its gray matter, and 

 the amount of its commissural fibres, there exists an enormous difference be- 

 tween the cerebrum of man and that of the highest anthropoid ape. There 

 is not, indeed, in these respects, so great a difference between them, as exists 

 between the lowest and the highest of the so-called Quadrumana ; but there is 

 a vastly greater difference than is found between the brains of any two quad- 

 rumanous species, or even between the brains of the different genera. Recent 

 researches on this subject, whilst they have served to show a closer affinity, 

 than was before believed to exist, between the Quadrumana and Man, still leave 

 a wide and unbridged chasm between them ; nor do geological researches yet 

 offer any intermediate and progressively approximating cranial forms. 



Of the several plans of the cerebral convolutions of the Mammalia, thus 

 briefly sketched out, it is difficult, at present, to say that the Pachydermatous 

 and Ruminant plan is necessarily inferior to the Carnivorous plan ; but there 



