LOCALISATION IN THE CORTEX CEREBRI. 723 



their size and grouping. 1 That such nervous impulses are normally set 

 agoing by impulses derived from the periphery through sensory tracts, 

 or from other regions of the brain through internuncial fibres, is certain, 

 but this acknowledged fact does not justify one in terming the areas in 

 question " sensory," for the typically motor cells of the anterior horn of 

 the spinal cord are also called into activity by impressions reaching 

 them from sensory tracts, and these are by common consent spoken of 

 as motor cells. The application of the terms motor and sensory to 

 cells in the central nervous system must be used rather for purposes 

 of convenience than with a view to a rigid definition of function. If we 

 define a motor or efferent cell as one which discharges towards a 

 " lower level " or towards the periphery, a sensory or afferent cell as 

 one which discharges towards the nerve centres or towards a higher 

 level, it is still difficult, if not impossible, in the present state of our 

 knowledge of the central nervous system, to say with regard to some 

 cells whether they are to be regarded as discharging towards higher or 

 lower levels. Nevertheless, when we find that excitation of the cells in 

 a limited region of the cortex produces movements which are in- 

 distinguishable from voluntary movements of the muscles, and that 

 extirpation of the same region produces permanent paralysis of those 

 movements without permanent or it may be without appreciable loss 

 of sensibility in the part affected, it appears not only convenient but 

 imperative to think and speak of the region in question as the centre 

 for the voluntary movements involved. 2 And the more so because 

 there are other regions of the cortex which, when artificially excited, 

 produce special movements of a voluntary character, the power to 

 execute which is, however, in these cases not lost on removal of the 

 cortex of the regions in question, although a definite sensory defect (in 

 the case of the occipital lobe, blindness) results. In spite, therefore, of 

 the fact that movements have resulted from their stimulation, we are 

 not justified in terming these portions of the cortex " motor," but may 

 regard them as " sensory," and may look upon the movements as being- 

 set up by a " motor " discharging centre elsewhere, as the result of 

 nervous impulses reaching it from the " sensory " region of the cortex. 



In connection with this subject, the " isolation experiments " which 

 have been performed by various observers 3 deserve notice. These have 

 consisted in cutting off the motor area from the rest of the cortex by 

 an encircling incision passing through the grey matter and some little 

 way into the white matter. In this manner it is believed that all fibres 

 connecting this area with other portions of the cortex are severed, and 

 only the fibres which pass in the corona radiata to or from the lower 

 centres remain. If these fibres are both afferent and efferent, — sensory 



1 Betz, Centralbl. f. d. med. Wissenseh. , Berlin, 1874 and 1881 ; Bevan Lewis, Proe. 

 Roy. Soc. London, 1878 ; and Brain, London, 1879, vol. i. p. 83. 



2 " If we are to retain the terms sensory and motor cells, it would seem more reasonable 

 to adhere to the designation "motor" for those cells which lie in the region of the cortex in 

 which the first steps in the so-called voluntary motor processes begin."— C. K. Mills, 

 "Nervous System and its Diseases," Philadelphia and London, 1898, p. 342. 



3 Exner and Vareth, Arch.f. d. ges. Physiol., Bonn, 1885, Bel. xxxvii. S. 523; ibid., 

 1887, Bd. xli. S. 349 ; ibid., 1889, Bd. xliv. ; Manque, " Rech. exper. sur le mecanisme 

 de fonctionnenient des centres psycho-moteurs du cerveau," Bruxelles, 1885 ; see also 

 Brain, London, 1885, vol. viii. p. 536. In some of Marique's experiments severance of the 

 motor centres of the dog, by a cut involving only the association tilires connected witli 

 the posterior parts of the brain, was found to produce almost as marked a motor paralysis 

 as complete isolation. I have found, however, that this is not the case in the monkey, nor 

 did the cutting off in the same individual of the prefrontal area as well produce paralysis. 



