758 A MANUAL OF PHYSIOLOGY 



So far, at least, as the Rolandic region and the grey matter imme- 

 diately around the neural canal are concerned, the analogy of an 

 electrical switch-board connected with machines of various kinds 

 might be more correct. Touch one key or another, and an engine 

 is set in motion to grind corn, or to saw wood, or to light a town. 

 The difference in result lies not in any difference of material or 

 workmanship in the switches, but solely in the difference in their 

 connections. 



Grey matter in the upper part of the Rolandic cortex is excited, 

 and the muscles of the leg contract. Grey matter around the lower 

 part of the fissure is excited, and there are movements of the face 

 and mouth. Grey matter in the medulla oblongata is excited, 

 and the salivary glands pour forth a thin, watery fluid, poor in 

 proteids, and containing an amylolytic ferment. Another portion of 

 grey (?) matter in the medulla is thrown into activity, and the pan- 

 creatic ducts become flushed with a thick secretion, rich in proteids 

 and in ferments which act on proteids, starch, and fat. Here, too, 

 there is a variety in result according as one or another nervous 

 switch is closed; here, too, the variety is due, not to essential 

 differences in the structure or the activity of the nervous centres, but 

 to their connection, by nervous paths, with peripheral 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 pro- 

 duced 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 specific 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 by the natural or homologous stimulus of light. Since a 

 similar mechanical stimulus applied to the auditory nerve gives rise 

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

 sensation of pain, many physiologists have assumed that the impulses 



