THE NERVOUS SYSTEM 349 



receivers and distributors of sensory impulses ; although a study 

 of the cerebral cortex does not justify the conclusion that they 

 are necessary links in its sensori-motor arcs. Large pyramids are 

 occupied with the nutrition of fibres which have a long traject 

 through the system. Hence they are " motor." They constitute 

 a marked feature of the area which is susceptible to stimulation. 

 They occur also in the visual area and elsewhere. Small 

 pyramids are associational ; that is to say, their axons do not 

 leave the cerebral hemispheres. They distribute impulses from 

 sensory areas to association zones, and from one part of an asso- 

 ciation zone to another. The layer of polymorphous cells is 

 relatively thicker in animals in which the cortex of the brain 

 exercises less control over action than in animals in which the 

 cortex is supreme in a rabbit thicker than in a monkey ; in a 

 monkey thicker than in Man. This layer is therefore said to be 

 concerned with the lower functions of the cortex, whatever this 

 expression may mean. Since the relative abundance of small 

 pyramids is a test of the supremacy of the cortex, we may speak 

 of them vaguely as concerned with its higher functions. But a 

 surer test of the capacity of the cortex for the elaboration of the 

 raw materials of thought which sensory nerves deliver to it is 

 the relative abundance of the tissue which intervenes between 

 its cells. The number of cell-bodies to be counted in a square 

 millimetre of a section of a given thickness is smaller in Man 

 than in a monkey, in a monkey than in a dog, and in a dog than 

 in a rabbit. 



A comparison of the brains of various mammals in which 

 particular sense-organs are either deficient or exceptionally 

 well developed affords the clearest proof of the localization of 

 sensory areas. This, if it were possible to make satisfactory 

 measurements, would be by far the best class of evidence as 

 to the part played by the several senses in an animal's mental 

 life. Unfortunately, measurement appears to be out of the 

 question ; but a glance at a rabbit's brain, placed by the side 

 of a mole's, shows that vision is localized in the occipital 

 region. All marine mammals are destitute of the sense of smell ; 

 the brain of a dog, compared with that of a porpoise or a 

 whale, shows that the sphenoidal region (cf. Fig. 25) is 

 associated with this sense. The brain of an otter exhibits 

 very clearly the area into which impulses arising in 



