212 FUNCTIONS OF THE NERVOUS SYSTEM. 



itself. In either case, the paralysis occurs in the opposite side of the body, as 

 we should expect in the decussation of the pyramids ; but it may occur either 

 in the same or on the opposite side of the /ace the cause of which is not very 

 apparent. If convulsions accompany the paralysis, we may infer that the 

 Corpora Quadrigemina, or the parts below, are involved in the injury ; and, 

 in this case, it is usually found that the convulsions are on the paralyzed side 

 of the body the effect of the lesion, both of the Cerebrum and of the Corpora 

 Quadrigemina, being propagated to the opposite side by the decussation of the 

 pyramids. Where, as not unfrequently happens, there is paralysis of one 

 side, accompanying convulsions on the other, it is commonly the result of a 

 lesion affecting the base of the Brain and Medulla Oblongata, on the side on 

 which the convulsions take place. Here the effect of the lesion has to cross 

 from the Brain, whilst its influence on the Medulla Oblongata is shown on the 

 same side. Many apparent anomalies present themselves, however, which 

 are by no means easy of explanation in the present state of our knowledge. 



288. The general result of such investigations is, that the Cerebrum is the 

 organ through which all those impressions are received which give rise to 

 Voluntary actions ; and that it affords the power of occasioning muscular 

 contraction, in obedience to the influence of the will : but that the fibres com- 

 posing its medullary portion are not susceptible of being thrown into action 

 by mechanical irritation, in the same manner as are those of the Spinal Cord 

 and Nerves, a peculiarity which may, perhaps, be connected with the differ- 

 ence of their structure, formerly explained ( 110). There is no positive 

 reason for the belief, that the Cerebrum is essential to the purely Emotional 

 actions ; and analogy, as we have seen, applied to the explanation of patho- 

 logical phenomena, would lead to the belief that their channel is different. 

 It can scarcely be denied, however, that in the Cerebrum resides that power 

 by which the attention of the mind is directed to any sensation; and by 

 which, through the medium of a brief reasoning process, a notion is formed 

 regarding its nature : this operation is altogether designated as perception, 

 which term, however, is also applied to its result. Now it will be presently 

 seen, that the formation of such elementary notions in us, is often a complex 

 process, though a rapid one ; whilst, in many of the lower animals, it appears 

 to be very much simpler, as to all those points, at least, which concern the 

 instinctive actions necessary for their well-being. Such intuitive perceptions 

 occasionally take place in ourselves ; but it will probably appear, from exami- 

 nation of them, that they are connected either with the mere Instincts, or with 

 the Emotions. 



289. Some metaphysicians have confounded Perception with Sensation ; 

 but the difference may be easily made evident. In order that a sensation 

 should be produced, a conscious state of the mind is all that is required. Its 

 whole attention may be directed towards some other object, and the sensation 

 calls up no new ideas whatever ; yet it will produce some change in the 

 sensorium, which causes it to be (as it were) registered there for a time, and 

 which may become the object of subsequent attention ; so that, when the 

 mind is directed towards it, that idea or notion of the cause of the sensation 

 is formed which constitutes a perception. For example, a student, who is 

 directing his thoughts to some object of earnest pursuit, does not receive any 

 intimation of the passage of time from the striking of a clock in his room. 

 The sensation must be produced, if there be no defect in his nervous system. ; 

 but it is not attended to, because the mind is bent upon another object. It 

 may produce so little impression on the mind, as not to recur spontaneously, 

 when the train of thought which previously occupied the mind has been 

 closed, leaving the attention ready to be directed to any other object ; or, the 

 impression having been stronger, it may 'so recur, and at once excite an idea 



