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.Course of the Paths of Nervous Conduction 108 Principles of Physiological Psychologyfunction which follows upon their removal.For the rest, it is a significant fact for the theory of thesephenomena of centromotor abrogation that they do not consist by any means in complete muscular paralyses.In general, there is inhibition of voluntary movement only: the muscles involved will still contract reflexwiseupon stimulation of the appropriate points upon the skin, and may be thrown into sympathetic activity by themovement of other muscle groups.Further, all symptoms of abrogation, save where very considerableportions of the cortical investment of both hemispheres have been removed, are impermanent and transitory;the animals will, as a rule, behave, after the lapse of days or months, in a perfectly normal way, and therestoration occurs the more quickly, the smaller the extent of the cortical area destroyed.[33]The demonstration of the centrosensory areas, if it is to be accurate and reliable, must, as we said above, beundertaken by help of the phenomena of abrogation.This limitation of method, and more especially theuncertainty which attaches to sensory symptoms, place serious obstacles in the path of investigation.Thereare, however, two points in which the disturbances of sensation set up by extirpations of the cortex in the dogappear to resemble the motor paralyses which we have already passed under review.First, the cortical regionscorrelated with the various sense departments are, evidently, not well-marked and circumscribed; theyalways cover large areas of the brain surface, and even seem to overlap.Secondly: the disturbances, here asbefore, do not consist in any permanent abrogation of function.If the injury is restricted to a comparativelysmall area, they may be entirely compensated.If it affects a larger portion of the cortex, there will, it is true,be permanent sensory derangements, but they will express themselves rather in an incorrect apprehension ofsense impressions than in absolute insensitivity to stimulus.Thus, dogs whose visual centre has been entirelyremoved will still avoid obstructions, and others, whose auditory centre has been extirpated, will react tosudden sound impressions, although they can no longer recognise familiar objects or the words of theirmaster.They take a piece of white paper, laid in their path, for an obstacle which they must go round; orconfuse bits of cork with pieces of meat, if the two have been mixed together.[34] All these phenomenaindicate that the functions of perception have in such cases been abrogated or disturbed, but that the removalof the centrosensory areas is by no means and in no sense the equivalent of destruction of the peripheral senseorgans.There is, further, one respect in which the terminations of the sensory conduction paths differ fromthose of the motor: while the derangements of movement point to a total decussation of the motor nerves,the disturbances of sensation, or at least of the special senses, are bilateral, and accordingly suggest that thefibres of the sensory paths undergo only a partial decussation in their course from periphery to centre.Figg.82, 83 and 84 show roughly the extent of the visual, auditory and olfactory areas in the cortex of the dog, asdetermined by the method of abrogation.The frequency of the dots indicates, again, the relative intensity ofthe disturbances which follow upon extirpation of the area in question; the black dots correspond to crossed,the hatched dots to uncrossed abrogation symptoms.We notice that the visual centre is situated for the mostpart in the occipital lobe, though less marked disturbances may caused from a part of the parietal lobe andprobably also from the hippocampus; the temporal lobe, on the other hand, is practically exempt.Theauditory area has its centre in the temporal lobe, from which it appears to extend over a portion of the parietallobe, as well as the callosal gyre and the hippocampus
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