700 EXTIRPATION OF THE MOTOR CENTRES. 



[In man, records of destructive lesions of the motor areas in whole or part have 

 now accumulated to such an extent as to leave no doubt, that if there be, say, a 

 destructive lesion of the middle third of the cortex of the ascending frontal and 

 ascending parietal convolutions, there will be paralysis of the arm of the opposite 

 side ; and* the same is true for the other centres.] 



[In extirpation or ablation of the motor centres, again, much confusion 

 has arisen from comparing the results obtained on different animals. In the dog, 

 there is no permanent motor paralysis, in the monkey and man there is. The 

 difference is this, that in the dog the lower centres, perhaps the basal ganglia, are 

 able to subserve the execution of those co-ordinated movements required for stand- 

 ing, progression, &c. As we proceed higher in the animal scale, the motor cortical 

 centres assume more and more of the functions subserved by the basal ganglia in 

 lower animals. There is, as it were, a gradual displacement of motor centres to 

 the cortical region, as we ascend in the zoological scale.] 



Differences in Animals. The higher the development of the intelligence of animals, the 

 more have their movements been learned, and the more have they gradually come to be con- 

 trolled by the will ; in them the disturbance of the motor phenomena becomes more pronounced 

 and persistent after destruction of the cortical psychomotor centres. Whilst in the lower 

 vertebrates, including the birds, extirpation of the whole hemispheres does not materially 

 interfere with movemeuts, the co-ordinated reflex movements being sufficient in dogs 

 occasionally, but exceptionally, extirpation of several motor areas produces visible permanent 

 disturbance of motor acts and in monkeys and man ( 378), the paralytic phenomena may be 

 intense and persistent. 



Acquired Movements. Among the movements performed by men are many which have been 

 acquired after much practice, and have been subjected to voluntary control, e.g., the move- 

 ments of the hands for many manual occupations. After alesion of certain motor areas, such 

 movements are reacquired only very slowly and incompletely, or it may be not at all. [The 

 interference with these finer acquired movements sometimes becomes very marked in lesions of 

 the motor areas produced by haemorrhage, and in some cases of hemiplegia.] Those move- 

 ments, however, J which are, as it were, innate [or as they are sometimes termed fundamental 

 in opposition to acquired], and are under the control of the will without much practice such 

 as the associated movements of the eyes, face, some of those of the limbs are either rapidly 

 restored after the lesion, or they appear to suffer but slightly after a lesion of the cerebral cortex; 

 the facial muscles are never so completely paralysed as from a lesion of the trunk of the facial 

 nerve ; usually the eye can be closed in the fonner case. The movements necessary for sucking 

 have been performed by hemicephalic infants. 



Theoretical. Hitzig ascribes the disturbance of movement, after the removal of the motor 

 centres, to the loss of the " muscular sensibility." Schiff refers it to the loss of tactile sensibility. 

 According to Ferrier, the tactile and sensory impressions are not appreciably diminished or 

 altered. The descending degeneration of the pyramidal tracts in the lateral columns, according 

 to Schiff, occurs after section of the posterior half of the cervical spinal cord, or even after 

 section of the posterior part of the lateral columns. After dividing the latter, and allowing 

 secondary degeneration to take place, it is not possible to discharge movements by stimulating 

 the cortex cerebri. [Schiff divided the posterior column of the cord, and found that stimula- 

 tion of the opposite motor cortex failed to excite movements in the opposite fore limbs. He 

 supposed that this result was due to ascending defeneration. Horsley finds, however, that 

 SchitTs results are due to transverse aseptic myelitis at the seat of operation, thus causing a 

 "block" there in the motor tract] The posterior columns, and their continuation upwards 

 to the brain, are supposed to carry the impulses upwards to the cerebrum (ascending the 

 limb of the reflex arc), where, after being modified in the centres, they are carried outwards by the 

 pyramidal tracts (descending limb of the reflex arc). [Some hold that the posterior columns 

 are directly connected with the cortical motor area, while others think that a sensory perceptive 

 centre is interposed between the afferent and efferent impulses.] Between, but deeper in the 

 brain, lie the centres for tactile sensibility. Landois and Eulenburg observed in a dog, from 

 which the motor centres for the extremities had been removed on both sides, that the move- 

 ments became completely ataxic, <., the animal could not execute such co-ordinated move- 

 ments as walking, standing, Ac. Goltz regards the disturbances of movement after injury of 

 the cortex as due to inhibition. Schiff maintains that when the cortex cerebri is stimulated we 

 do not stimulate a cortical centre, but only the sensory channels of a reflex arc, the continua- 

 tion of the posterior columns, so that on this supposition the movements resulting from 

 stimulation of the motor points would be reflex movements. The centres lie deeper in the 

 brain. This view is not generally entertained. 



Modifying Conditions. The excitability of the motor centres is capable of 



