NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHALON.) 



711 



sound hemisphere may excite to action the 

 centre of volition (corpus striatum) on the dis- 

 eased side. 



The existence of hemiplegic paralysis, then, 

 implies an affection, direct or indirect, of the 

 centre of volition (corpus striatum) on the op- 

 posite side. Pressure, or a morbid change in 

 the physical state of its tissue originating in it 

 or propagated to it, is all that is necessary for 

 tins purpose; and this change, like the change 

 in the normal actions, may be of such a kind 

 as to elude our means of observation. 



When a sensation is excited, the stimulus 

 acts from periphery to centre. The change is 

 propagated by the sentient nerve to the optic 

 tlialamus, which, by its numberless radiations 

 and its many commissures, is well calculated 

 to excite all parts of either hemisphere, and 

 even of both hemispheres. When the nerve 

 excited is one of pure sense, the change is 

 wrought more directly in the brain; if the fifth, 

 or any of the nerves of the medulla oblongata, 

 the stimulus acts directly on the part ; but if a 

 nerve of either limb be stimulated, the change 

 must be propagated through the spinal cord. 



It will be asked, if this be the modus ope- 

 rundi in sensations, how does it happen that 

 disease of one optic thalamus does not impair 

 sensation in one-half of the body? And how 

 is it that such disease is much more frequently 

 associated with hemiplegic paralysis, of a kind 

 not to be distinguished from that which de- 

 pends on diseased corpus striatum. The answer 

 to the first question is as follows. The optic 

 thalamus, or, more properly, the centre of sen- 

 sations, is never wholly diseased, for this centre 

 is not confined to the optic thalamus of descrip- 

 tive anatomists, but extends to the mesocephale 

 and olivary columns. Extensive disease of 

 this centre would probably be fatal to sensa- 

 tion. But the most ample provision exists for 

 opening up new channels of sensation if those 

 on one side or a part of them be impeded. 

 The centres of opposite sides are intimately 

 connected, especially in the medulla oblongata 

 and mesocephale, bycommissural or by decus- 

 sating fibres ; the optic thalami of opposite 

 sides are connected to each other by the poste- 

 rior commissure and the soft commissure, and 

 the immense multitude of fibres which radiate 

 from each thalamus insure its connection with 

 a considerable extent of the brain, so that a 

 change in any part of it cannot fail to be com- 

 municated to some portion of the hemisphere. 

 It is sufficient for mere sensation that the centre 

 of sensibility should be affected. Intellectual 

 change resulting from that affection depends 

 upon fibres which radiate between it and the 

 optic thalami. 



1 1 often happens that at the onset of a cerebral 

 lesion sensation as well as motion is paralysed 

 in the opposite side of the body. In a few days, 

 however, the sensibility returns whilst the pa- 

 ralysis of motion remains, a fact which is 

 sufficient to show that the motor and sensitive 

 power must have different channels in the 

 centres as well as in the nerves. The primary 

 paralysis of sensation may be due to a lesion 

 on one side affecting the centre of sensibility, 



or to the shock which that centre may have 

 received from the sudden occurrence of lesion 

 in some other neighbouring part. In the latter 

 instance the recovery of sensibility takes place 

 evidently on the subsidence of the effects of 

 shock : in the former it may depend on the 

 existence of other channels of sensitive impres- 

 sions, independently of those involved in the 

 lesion. Hence there may be lesion of one 

 optic thalamus without loss of sensibility. 



The answer to the second question is ob- 

 tained from considering the intimate connection 

 of the corpus striatum and optic thalamus. 

 No two parts of the brain are so closely united 

 by fibres in vast numbers passing from one to 

 the other. Disease of the thalamus therefore 

 may excite a morbid state of the corpus 

 striatum, without producing any change in its 

 structure, which may be recognised by the or- 

 dinary means of observation. And thus he- 

 miplegia will take place, and remain as long 

 as the morbid state of the corpus striatum re- 

 mains. A lesion of the corpus striatum may 

 in a similar manner affect the optic thalamus 

 of the same side ; but as that is not the only 

 channel of sensitive impressions, a loss of sen- 

 sibility does not necessarily occur. 



Emotions are for the most part excited 

 through the senses. A tale of woe, a dis- 

 gusting or painful spectacle, a feat of won- 

 derful power or skill, the sudden appearance 

 of a person not expected, are calculated to 

 produce corresponding emotions of pity, dis- 

 gust or pain, wonder or surprise. But emo- 

 tions may likewise be produced by intellectual 

 change. The %vorkings of the conscience 

 may remind one of some duty neglected or 

 some fault committed, and the emotion of 

 pain, or pity, or remorse may ensue. Now 

 emotion may give rise to movements indepen- 

 dently of the will. The extraordinary influ- 

 ence of emotion on the countenance is well 

 known, and this may affect one side of the 

 face, which is paralysed to the influence of the 

 will, or it may excite movements of the limbs, 

 even when the will can exert no controul over 

 them. From these facts it is plain that that 

 part of the brain which is influenced by emo- 

 tion must be so connected that the convolu- 

 tions may affect it or be affected by it; that it 

 may be readily acted on by the nerves of pure 

 sense; that it may influence the spinal cord 

 and the motor nerves of the face when the 

 ordinary channels of voluntary action have been 

 stopped. No part possesses these conditions 

 so completely as the superior and posterior part 

 of the mesocephale, which we have already 

 noticed as concerned in acts of sensation. Is 

 an emotion excited by an impression made 

 upon one of the senses ? this part becomes 

 directly affected, and through the optic thala- 

 mus the emotional feeling causes intellectual 

 change. The working of the intellect on the 

 other hand may act on the seat of emotion 

 through the same channel. And an excite- 

 ment of this part may produce movement of a 

 limb, or of all the limbs, through its influence 

 on the spinal cord through the olivary columns. 

 The cerebellum influences the antcro-lateral 



