THE CENTRAL NERVOUS SYSTEM 759 



^set up in the auditory nerve when sound impinges on the tympanic 

 membrane do not differ essentially from those set up in the optic 

 nerve when a ray of light falls upon the retina, or from those set 

 up in the fifth nerve by the irritation of a carious tooth, or from 

 those set up in certain fibres of all cutaneous nerves when a warm 

 body comes in contact with the skin. Since the results in conscious- 

 ness are very different, this assumption has necessitated the further 

 conclusion that somewhere or other in the central nervous system 

 there exist 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, there is no reason to suppose that the 

 nerve-impulses which travel up to them are absolutely similar until 

 they have reached the centres, and there suddenly become, or pro- 

 duce, 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 end organs is essential in order that sensations of touch and tem- 

 perature should be experienced. Such facts indicate that the afferent 

 impulses are 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 auditorynerve-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 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 



