NERVOUS SYSTEM. VERTEBRATA. 355 



relatively to the size of its enormous body, it is the smallest 

 mammalian brain. Thus, according to one record, the 

 brain of a Common Rorqual weighed 4700 grammes and 

 the body of the animal, approximately, 25,000 times as 

 much. 



From recent investigations as to the factors which deter- 

 mine the size of the brain in mammals, there can be little 

 doubt that in any given Order of mammals of the same 

 geological epoch the size of the cerebral cortex varies directly 

 with the extent of the sensory surfaces of the body. It 

 must, however, be borne in mind that the impressionable 

 surfaces of the organs of the special senses have a relatively 

 greater representation in the cortex than the general sensory 

 areas, because the psychical importance of the olfactory, 

 visual, and auditory impressions is greater than that of 

 mere tactile sensibility. In the Whale the olfactory areas 

 become reduced to insignificant proportions. The diameter 

 of the retina of Sibbald's Whale is only 2^ times as great 

 as that of the Porpoise (Dubois, op. cit. p. 356). So that, 

 presumably, the visual area of cortex, although much larger 

 in the Whale than in the Porpoise, is relatively much 

 smaller in the former than in the latter. Then, again, the 

 disproportion between the general surface areas of the 

 Porpoise and Whale is much greater than that of the body- 

 weight. Hence the brain of a small animal (the Porpoise) 

 is much greater than that of a large animal (the Whale) 

 per unit of body-weight. 



For a given unit of sensory surface the extent of the 

 cerebral cortex varies in different Orders, or, as Dubois puts 

 it, " the index o cephalisation increases as the organisation 

 of the animal becomes higher." Thus, in a series of esti- 

 mations of this index, Dubois gives 2*8186 and 2*6778 as 

 the human male and female figures respectively ; for the 

 Apes indices ranging from 0*3636 (Macacus) to 0*7607 

 (Hyldbates) ; for the Carnivores from 0*2166 (Putorius) to 

 0*4413 (Lycaon very old specimen); for the Ungulates 

 from 0*2026 (Tapir) to 0*4573 (Equus), sinking in ex- 

 ceptional cases as low as 0*1819 (Hippopotamus) and rising 

 as high as 1*2484 (Elephas): and finally for the Rodents 

 from 0*0767 (Mus) to 0*2369 (Hydrochcerus). Estimating 



2 A2 



