500 PHYSIOLOGY 



they are unable to carry out even the simpler motor reactions of the 

 body after removal of those portions of the hemispheres especially 

 engaged in the control of voluntary movement. The motor 

 defect therefore which is brought about in man, as a result of 

 destruction of one or more of the motor centres, is to a large extent 

 permanent. 



If the lesion in man be strictly limited to the motor areas in the 

 ascending frontal convolution, it is impossible to detect any loss of 

 sensation in the affected parts of the body. On the other hand, some 

 loss of sensation is often found where the paralysis is widespread and 

 occasioned by extensive lesion in the neighbourhood of the Rolandic 

 area. Moreover, even in localised lesions in man, an epileptic fit may 

 be preceded by a sensory aura in the part which is the starting-point 

 of the convulsive movements. Much discussion has taken place as 

 to the exact significance to be assigned to these slight sensory pheno- 

 mena. By some observers, e.g. Munk, it has been thought that the 

 motor centres were the end-stations of the fibres subserving muscular 

 sensations, and that the movements resulting from their stimulation 

 were due to the revival of such sensations. Bastian insisted on the 

 important part played in voluntary actions by afferent impressions, and 

 these centres have sometimes been spoken of as ' kin aesthetic ' or sensori- 

 motor. The discussion has, however, now resolved itself practically 

 into one of terms. There is no doubt that, when the lesion is strictly 

 localised in the motor area, paralysis may be present without any 

 loss of sensation whatsoever. The paralysis therefore cannot be 

 classed with the sensori-motor paralysis distinguished earlier as the 

 result of division of sensory roots. On the other hand, when we say 

 that this part of the brain represents a ' centre for voluntary move- 

 ments,' we do not mean that the volitional motor impulses arise de 

 novo from the pyramidal cells in its grey matter. Every neuron in the 

 nervous system is part of an arc, and it is generally difficult to label any 

 given neuron as definitely sensory or motor. In a reaction involving 

 a chain of neurons we can assign the name of motor to that 

 neuron which sends its axon to the muscle, and of sensory or afferent 

 to that neuron which receives the impulses at the periphery of the 

 body. Where in the chain we are to draw the dividing line and to 

 say these neurons are sensory and those motor, it is difficult to decide. 

 The motor areas in the cortex give origin to the long fibres of the pyra- 

 midal tract, which passes right through the central nervous system 

 to the segmental centres of the cord. We know that the integrity of 

 these tracts is essential for the carrying out of voluntary movement. 

 It is therefore convenient to speak of them as motor or efferent tracts, 

 and their origin as motor centres ; although these tracts have the 

 s^me relation to the motor-cells of the spinal segment as have the 



