870 A MANUAL OF PHYSIOLOGY 



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 precentral convolution is 

 excited, and the muscles of the leg contract. Grey matter on the 

 lower part of the convolution 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 proteins, and containing an amy lory tic ferment. Another portion 

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

 pancreatic ducts become flushed with a thicker secretion, relatively 

 rich in proteins and in ferments which act on proteins, 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 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 phrase- 

 ology, 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 or adequate 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 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 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 



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



