THE CENTRAL NERVOUS SYSTEM 871 



organs that are differently affected by the same kinds of afferent 

 impulses in other words, that sensory localization is at the centre. 

 On this view, the visual areas in the cortex respond to all kinds of 

 stimuli by visual sensations ; the auditory areas by sensations of 

 sound, and so on. 



But while it cannot be doubted that special sensory regions exist 

 in the grey matter of the brain, where the afferent paths concerned 

 in the different kinds of sensation end, there is no reason to suppose 

 that the nerve-impulses which travel up the various paths are 

 absolutely similar until they have reached the centres, and there 

 suddenly become, or produce, sensations absolutely different. There 

 is, indeed, evidence of a certain amount of sensory specialization at 

 the periphery. For example, when an ordinary nerve-trunk is 

 touched, the resultant sensation is not one of touch. If there is any 

 sensation at all, it is one of pain. Heating or cooling a naked nerve- 

 trunk gives rise to no sensations of temperature. When the ulnar 

 nerve is artificially cooled at the elbow, the first effect is severe pain 

 in the parts of the hand supplied by the nerve. The pain disappears 

 somewhat abruptly as cooling goes on, and is succeeded by gradual loss 

 of all sensation in the ulnar area of the hand ; but the cooling of the 

 nerve-trunk does not give rise to any sensation of cold (Weir Mitchell) . 

 Stimulation of the receptors or end-organs is normally essential in 

 order that sensations of touch and temperature should be experienced 

 (but see p. 980) . 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 sensation 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 discriminated 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 experi- 

 ment, 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 specializa- 

 tion 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 of 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. 



