324 SPECIAL PHYSIOLOGY. 



can be no doubt, that this latter is decidedly lower than the plan observed in 

 the Primates, which include the so-called Quadrumana and Man. 



In regard to the relative complexity of different brains within each plan, a 

 very general rule has been observed (though exceptions to it have been pointed 

 out), viz., that the cerebral hemispheres are more convoluted in the larger 

 species or genera of any great group of the Mammalia, than in the smaller 

 species or genera of such groups. For example, this is true successively of 

 the larger as compared with the smaller Quadrumana, as seen in the orang, 

 the baboons, the monkeys, and the lemurs ; of the larger and smaller Carniv- 

 ora, as in the seal and the cat ; of the larger and smaller Pachydermata, as in 

 the elephant, horse, and pig ; of the larger and smaller Ruminants, as in the 

 ox and sheep, and so on. The case of the elephant is perhaps the best single 

 known illustration of the striking relation between the size of the body and the 

 complexity of the cerebral convolutions, which are singularly numerous and 

 tortuous in that large animal. They are also very complex in the somewhat 

 allied, and usually bulky, Cetacea. As an exception to the general rule, it is 

 stated that the brain of the horse is less complicated than that of the ass, 

 although the former animal is larger ; but the pony's brain is certainly more 

 complex than the donkey's. Again, the brains of the lion and cat, notwith- 

 standing the difference in size between their bodies and their brains, also pre- 

 sent none in the degree of complexity of their cerebral convolutions. It has 

 been suggested, that the relatively more convoluted cerebrum of the larger 

 species, is to enable the necessary amount of gray matter to be contained in a 

 cranium of a given size, otherwise, the head would have been inconveniently 

 bulky (Dareste) ; but this is probably not the whole explanation, or there 

 would be no exceptions. Besides this, the cranial cavities of the elephant and 

 whale are not nearly so large as their heads would allow. However, it has 

 been shown, that, although the effect of the convolution of the surface of the 

 cerebrum, is to increase largely the quantity of gray matter, yet the size or 

 weight of this organ by no means increases, pari passu, with the complication 

 of its surface ; for, in proportion to its surface, which is so highly convoluted, 

 the cerebrum of man is only two-and-a-half times as large as that of the rabbit, 

 the surface of which is quite smooth (Baillarger) ; the larger quantity of me- 

 dullary commissural fibres in the superior brain, accounts for this. 



In brains still more simple than those of the lowest Mammalia, not only are 

 there no convolutions, but neither external nor internal distinctions into lobes. 

 A few symmetrical lines only are traceable in Birds, but none whatever in 

 Reptiles, Amphibia, and Fishes. The so-called cerebral lobes, or rather their 

 superficial layers, in these four oviparous classes of Yertebrata, are by some, 

 indeed, supposed to represent the anterior lobes only, of the hemispheres of 

 the cerebrum in the Mammalia ; first, from the absence of the corpus callosum, 

 and secondly, from their connection with other parts. The middle lobes are 

 believed to appear first, in the lower Mammalia, and afterwards, the posterior 

 lobes in the higher forms. But, as already intimated, the lobes may not be 

 distinguishable, and yet homologous parts of the cerebral hemispheres may be 

 present, however slightly developed, throughout all the Yertebrata. 



Amidst the known varieties of plan, and all the degrees of complication of 

 the cerebral convolutions within the limits of each plan, one particular feature 

 seems to be of considerable importance in estimating the relative superiority 

 of any given brain. We allude to the degree of symmetry of the convolutions 

 of the two hemispheres. In the simple, diminutive hemispheres of the Fish 

 and Reptile, even in the more highly developed, and slightly sulcated, hemi- 

 spheres of certain Birds, and in the smooth cerebra of the Monotremata and 

 Marsupialia, and of the lower Rodentia, and the lemurs, the symmetry of form 

 is apparently exact. As soon as any markings appear on the hemispheres, 

 and even when these are tolerably numerous, as in the Carnivora and more 

 highly developed monkeys, they are very symmetrical on the two sides ; but 

 in the more complex brains of the larger Pachydermata and Ruminantia, 

 especially in the horse and elephant, and also In the still more highly devel- 

 oped brains of the anthropoid apes, a certain want of symmetry becomes ap- 

 parent. But it is in the human brain more particularly, that exactitude of 

 symmetry disappears. In fact, the extraordinary relative size of the cerebral 



