944 THE CENTRAL NERVOUS SYSTEM 



natural or homologous or adequate stimulus of light. Since a similar 

 mechanical stimulus applied to the auditory nerve gives rise to a sensa- 

 tion of sound, and, applied to the trigeminal nerve, to a sensation of pain, 

 many physiologists have assumed that the impulses 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 irri- 

 tation of a carious tooth, or from those set up in certain fibres of the 

 cutaneous nerves when a warm body comes in contact with the skin. 

 Since the results in consciousness 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 viscual 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, it has not been proved 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 cf tempera- 

 ture. 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. 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 



