FUNCTIONS OP THE BRAIN 970 



paths, with peripehral organs of different kinds. There is, indeed, a 

 specialization, a localization, of function, but the localization is at the 

 periphery, the specialization is in the peripheral organs. 



It may be asked whether, if this is the case for the peripheral organs 

 of efferent nerves, the converse does not hold true for the afferent nerves 

 in other words, whether the localization here is not at the centre. And 

 that there is in some degree a central localization of sensation may be 

 considered proved by the well-known clinical fact, already referred to, 

 that sensations of various kinds may be produced by pathological 

 changes in the cortex. For example, a tumour involving the upper part 

 of the temporal lobe may give rise to epileptiform convulsions preceded 

 by an auditory aura, a sound, it may be, resembling the ringing of bells ; 

 a tumour involving the occipital region may cause a visual aura, and so 

 on. Central sensory localization is the fundamental idea of the old 

 doctrine of the specific energy of nerves, which, in modern phraseology, 

 expresses the fact that excitation of the central end of a sensory nerve 

 by various kinds of stimuli causes always or at least very often the 

 particular kind of sensation appropriate to the nerve. The observation 

 so frequently made in surgery before the days of anaesthetics, that when 

 the optic nerve was cut in removing the eyeball the patient experienced 

 the sensation of a flash of light,* was long looked upon as the strongest 

 prop of the law of specfic energy, and well illustrates the meaning of the 

 term. Here a mechanical excitation of the optic fibres in their course 

 gives rise to the same sensation as excitation of the retina bv the 

 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 hi 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 hi 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 of 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 



* It is said that this is not always the casa. 



