SENSES AND THE CENTRAL NERVOUS SYSTEM 247 



the Blue Whale represents 0-007 P^^ cent, that of the Fin Whale o -016 per 

 cent, that of the Humpback Whale 0-02 per cent, that of the Sperm Whale 

 0-03 per cent, that of the Pilot Whale 0-083 P^^' cent, that of the Bottlenose 

 Dolphin 0-225 P^'^ cent, that of the Common Dolphin o -666 per cent, that 

 of the porpoise 0-854 per cent, compared with the elephant's 0-12 per 

 cent, the horse's 0-154 per cent and man's i -93 per cent. In other words. 

 Rorquals with the biggest brains have the smallest brain to body-weight 

 ratio and porpoises with the smallest brain the highest ratio of all Ceta- 

 ceans (Fig. 127). From this, we can draw a general inference: the bigger 

 the animal the smaller the ratio of brain to body weight. 



This rule applies, by and large, to all animals - the brain of a cat 

 representing o -94 per cent of its body weight, and that of a lion a mere 

 0-18 per cent, etc., and can be explained by the fact that the relative size 

 of the brain is determined by the size of the animal's surface rather than 

 by its bulk, since the brain is intimately associated with the fvmctions of 

 a great many sensory organs, the receptors of which lie in a plane. More- 

 over, the surface of the skin is closely related to a given animal's metabolic 

 rate, which thus affects the size of the brain. Now as a given animal grows 

 larger, its surface area (and hence its brain) becomes smaller with regard 

 to its bulk (or weight) because the surface increases as the square and 

 the bulk as the cube of the increment. Hence the bigger the animal, the 

 larger its brain, but the smaller its brain to body-weight ratio. 



However, things are not quite as straightforward as that, as a brief 

 glance at Fig. 127 will show. The Sperm Whale which tops column a 

 does not take last place in column b as we might have expected it to do, 

 and man does not come last in column a, etc. Moreover, Crile and 

 Quiring, comparing the brains of a horse and a Beluga (two animals 

 which happened to have identical weights of 1,1501b.) found that the 

 former weighed i -78 lb., while the latter weighed 5-19113., though the 

 surface area, and probably also the sensory surface area of the Beluga 

 which, inter alia, has no olfactory epithelium, is undoubtedly smaller than 

 that of the horse. 



The Dutch expert, Prof. Dubois, who achieved world-wide distinction 

 through his discovery of Pithecanthropus, therefore established a formula 

 for relating brain-weight to body-surface. Using his formula, we find 

 that the brains of mammals fall into seven categories. Man, whose 

 brain to surface ratio is the highest, falls into the top category, i.e. the 

 seventh stage of cephalization ; Pithecanthropus occupies the sixth stage; 

 anthropoid apes the fifth stage ; other apes, most carnivores, and ungulates 

 the fourth stage; many rodents, including the rabbit, the third stage; mice 

 and hedgehogs the second stage; and shrews and bats the first stage. 

 Mysticetes would fall into the fourth and Odontocetes into the fifth stage 



