9 8o THE CENTRAL XERVOUS SYSTEM 



essential in order that sensations of touch and temperature should be 

 experienced. Although as previously stated, one great function of 

 the receptor is to lower the threshold of the adequate stimulus, 

 and thus to render the afferent neuron more easily excited by an 

 adequate stimulus than by any other, it may also serve to impress a 

 particular rhythm or other character upon the nerve impulse, so that the 

 afferent impulses may be to some extent differentiated before they reach 

 their centres. One reason, then, why excitation of the temporal cortex 

 by impulses falling into it along the auditory nerve-fibres causes a sensa- 

 tion different from that caused by impulses reaching the occipital cortex 

 through the fibres of the optic nerve may be a difference in the nature of 

 the impulses. If this were the only reason, it would follow that were it 

 possible to physiologically connect the fibres of the optic radiation with 

 the temporal cortex, and those of the temporal radiation with the 

 occipital cortex, sights and sounds would still be perceived and dis- 

 criminated in a normal manner, although now the integrity of the 

 occipital lobe would be bound up with the perception of sound, the 

 integrity of the temporal lobe with visual sensation. This state of 

 affairs would correspond to complete specialization for sensation in the 

 peripheral organs, complete absence of specialization in the centres. On 

 the other hand, it is conceivable that, after such an ideal experiment, 

 sound-waves falling on the auditory apparatus might cause visual 

 sensations, and luminous impressions falling on the retina sensations of 

 sound. This would correspond to complete specialization of sensation 

 in the centres, complete absence of specialization at the periphery. A 

 third possibility would be that the ' transposed ' centres, responding at 

 first feebly or not at all to the new impulses, might, by slow degrees, 

 become more and more excitable to them. This would correspond to a 

 peripheral specialization, combined with a tendency to development oi 

 central specialization. And, indeed, it is not easy to conceive in what 

 way, except as the result of differences in the nature of impulses coming 

 from the periphery, specialization of sensory areas in the central nervous 

 system could have at first arisen. 



Degree of Localization in Different Animals. Before leaving this 

 subject, two points ought to be made clear: (i) The degree of 

 localization of function in the cortex goes hand in hand with the 

 general development of the brain. In man and the monkey, the 

 motor localization is more elaborate than in the dog that is to 

 say, a greater number of movements can be associated with definite 

 cortical areas. In the rabbit, whose ' motor ' centres have been 

 particularly studied in recent years by Mann and Mills, localization 

 is still less advanced than in the dog. Towards the bottom of the 

 mammalian group certain ' motor ' areas can still be demonstrated, 

 though they are rather ill-defined, for instance in the hedgehog 

 (Mann), opossum (Cunningham), and ornithorhynchus (Martin). 

 In general the movements of the anterior limb are easier to obtain 

 than those of the posterior. In birds Mills found no evidence of the 

 existence of any ' motor ' centres. 



(2) Areas of the same name (homologous areas) in different groups 

 of animals do not necessarily have the same function that is, in 

 the case of the ' motor ' areas, are not necessarily associated with 

 the same movements. Taking the position of the centre for the 



