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NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPUALON.) 



are met with in the brain. The peculiar struc- 

 ture of particular parts, as the optic thalami, 

 corpora striata, tuber cinereum, &c. has been 

 already described. 



The grey matter of the convolutions of the 

 brain presents the same characters throughout, 

 excepting in certain convolutions of the poste- 

 rior lobe near the posterior and inferior horns 

 of the lateral ventricles. Here, we may ob- 

 serve, in a horizontal section, the grey matter 

 of the convolutions separated into two portions 

 by a delicate white line, well represented in 

 Jig. 398. This layer of light matter was first 



Fig. 398. 



\ \ '\ 



< White line in the yrey matter of convolutions of the 

 posterior lobe. 



described by Vicq d'Azyr, but has attracted 

 very little attention from subsequent anatomists. 

 I have never looked for it without finding it. 

 It consists of nucleated particles, similar to 

 those in the grey matter of the cerebellum. 

 The layer of grey matter external to it contains 

 few nerve-fibres ; that internal to it contains them 

 in great numbers, passing into it at right 

 angles. 



It is not intended in this part of the article 

 to discuss the physiology of the brain. But in 

 order to develope more clearly than can be 

 done in a mere description the connection of 

 its several parts and the views of its struc- 

 ture which 1 believe to have the best founda- 

 tion, I shall state briefly what appears to be the 

 probable modus operandi of the organ, whether 

 as the source of voluntary action or as the re- 

 cipient of sensitive impressions. 



It will be necessary first to state the follow- 

 ing propositions as postulates. 



1. The vesicular matter is the source of ner- 

 vous power. In mental actions it is the part 

 immediately associated with changes of the 

 mind : whether in the working of the intellect, 

 or in the exercise of the will, or in the per- 

 ception of sensitive impressions. 



2. The convolutions are the parts immedi- 

 ately concerned in the intellectual operation. 



3. The simple exercise of the will, for a 

 voluntary movement, is probably connected 

 with the corpora striata. 



4. The mere reception of sensitive impres- 

 sions is connected with the optic thalami and 

 the superior layer of the crus cerebri. 



5. Mental emotions affect the posterior and 

 superior part of the mesocephale. 



6. The cerebellum is the regulator of the 

 locomotive actions. 



These propositions, which, it is admitted, 

 although not improbable, are far from being 

 proved, will serve as the basis of an hypothesis 

 of the action of the brain. 



In simple operations of thought, as in the 

 exercise of the reasoning powers, or of those of 

 the imagination, the convolutions of the brain 

 are immediately engaged. We do not say that 

 material changes give rise to the mental actions, 

 but rather that the changes of the immaterial 

 mind and those of the vesicular matter of the 

 convolutions are simultaneous. 



If an intellectual act gives rise to the exer- 

 cise of the will, the change in the superficial 

 vesicular matter is propagated by the fibres of 

 the hemisphere to the corpus striatum, whereby 

 the will is excited, and the change in the vesi- 

 cular matter of that body is propagated along 

 the inferior layer of the crus cerebri, and, after 

 passing through the mesocephale, along the an- 

 terior pyramids to the spinal cord, each ante- 

 rior pyramid acting upon that antero-lateral 

 column of the cord which is on the opposite 

 side of the body to itself. 



The pyramids connect the vesicular matter 

 of the corpora striata with that of the spinal 

 cord ; from their small size it is highly impro- 

 bable that they can be viewed as continuations 

 of spinal nerves up into the brain. 



Simple solution of continuity of the fibres 

 of the hemispheres, which does not cause 

 pressure, nor affect in any way the corpora 

 striata, would therefore merely cut off the 

 communication between the seat of intellectual 

 action and the centre of voluntary action. The 

 will, although unaffected, is unable to keep 

 up with the train of thought, and mental con- 

 fusion is the result. The loss of speech, which 

 sometimes precedes a paralytic attack, and 

 which may remain even after the paralysis has 

 been removed, may be accounted for in this 

 way. The intellect is competent to shape 

 the thought, but unable to excite the will, 

 upon which the exercise of the organs of speech 

 is so obviously dependent. 



Changes, originating or excited in either 

 hemisphere, may be propagated to the corre- 

 sponding parts of the other hemisphere by the 

 transverse commissures, the corpus callosum, 

 anterior commissure, &c. How far both he- 

 mispheres are in simultaneous action, during 

 the rapid changes of the mind in thought, can 

 scarcely be determined ; it seems probable, 

 however, that, in certain acts of volition, one 

 only is the seat of the change which prompts to 

 the movement. If I will to move my right 

 arm, the change by which that movement is 

 prompted belongs to the left hemisphere and 

 corpus striatum. 



Certain cases of disease confined to one he- 

 misphere, in which a considerable degree, at 

 least, of intellectual power persists, denote 

 that the sound one may suffice for the manifes- 

 tation of the changes connected with thought, 

 and it may be reasonably supposed that the 



