442 PHYSIOLOGY 



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 phenomena. 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 ' kinsesthetic ' 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 dis- 

 tinguished 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 movements,' 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 pyramidal 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 move- 

 ment. It is therefore convenient to speak of them as motor or efferent 

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

 same relation to the motor- cells of the spinal segment as have the afferent 

 fibres from the posterior roots by which similar movements may be evoked. 



On the other hand, the activity of the pyramidal cells of the cortex, 

 like those of the motor- cells of the spinal cord, is determined by the arrival 

 at them of afferent impressions.' In the absence of these afferent impressions 

 no spontaneous discharge of motor impulses takes place. Thus in the spinal 

 frog we have seen that complete inactivity is brought about by section of 

 all the posterior roots. In the same way paralysis of the arm is induced 

 by section of all its posterior roots, although it can be shown that the motor 

 cortex is still excitable, and that the application of an induced current to the 

 motor centres of the arm evokes a movement as easily as in the normal 

 animal. The motor- cells in the cortical motor centres are normally played 

 upon and aroused by impressions arriving at them from all other parts of the 

 brain and nervous system, and determined originally by impressions falling 

 on the surface of the body. 



