724 THE CEREBRAL CORTEX. 



and motor, — there seems no reason why the motor centres should not 

 continue to act in response to tactile impressions. They do not, how- 

 ever, so act (although they are still capable of being excited by direct 

 electrical excitation), but the isolation produces a paralysis as complete 

 as if the centres in question were excised. The inference is that the 

 motor centres are not themselves directly provided with sensory excito- 

 motor fibres, but that they are set in action by impulses received by 

 means of association fibres from sensory parts of the cortex. 



Although this is the obvious inference, it must be pointed out that 

 the results are open to another interpretation, namely, that the cutting 

 off of the innumerable impulses which the motor centres are constantly 

 receiving from the rest of the cortex, has rendered them less liable to 

 respond to afferent impulses reaching them by the corona radiata. 

 Such an explanation presupposes the maintenance of a kind of tonus in 

 the motor centres of the cortex by impulses received through association 

 fibres. 



It must also be pointed out that these experiments are open to the criticism 

 that the circumsection of an area very materially interferes with the blood 

 supply of its cortex, and may thereby interfere also with its functional activity. 



While there can be no objection, if it is preferred, to the employ- 

 ment of the term " psycho-motor " in place of the more simple term 

 " motor," to denote the excitable centres in the Kolandic region of the 

 cortex cerebri, the use of the term " sensori-motor " is to be deprecated, 

 because it assumes, what has not yet been shown to be the case, that 

 the region in question possesses, besides the faculty of receiving im- 

 pressions from other portions of the cortex, and transmitting them as 

 voluntary impulses to the lower level efferent stations in the bulb and 

 cord, also that of perceiving sensory impressions directly received from 

 lower level afferent stations ; in other words, that it also functions as a 

 region devoted to general (tactile) sensibility. This view has, it is true, 

 been taken by Bastian, Dana, Flechsig, Mott, H. Munk, Schiff, Soury, 

 and other distinguished neurologists, 1 but as appears to me on insufficient 

 evidence. It has found its strongest expression in H. Munk's division of 

 the Rolandic area into so-called " sensory spheres" 2 (Fig. 330). Munk 

 regards the movements which are evoked by excitation of the Eolandic 

 region of the cortex as due entirely to the calling up of the sensations 

 which ordinarily accompany those movements, and that the paralysis 

 which results from extirpation of the parts in question is a paralysis of 

 sensation; just as when all the sensory nerves (posterior roots) of a limb 

 are cut, the power of voluntary movements, which are guided by 

 sensations derived from the periphery, is greatly impaired, or in some 

 cases appears to be entirely lost. In support of this view Munk asserts 

 that removal of the part of the cortex, excitation of which produces 



1 Schiff was the first physiologist to regard the Rolandic zone as a purely sensory region, 

 hut he afterwards saw reason to give up this idea, and in an addendum to his collected 

 papers (published in 1895) he definitely states that experiments have shown that motor 

 centres must exist in the cerebral cortex. The view originally taken by Hitzig was that the 

 paralysis obtained is caused by loss of muscular sense or of loss of the consciousness called 

 up by the imagining of movements ; later he admitted that loss of sensibility of tendons, 

 ligaments, and articulations might also be a cause {Neurol. Centralbl., Leipzig, 1888). 



2 Munk has enunciated his views upon this question in a series of articles in the Arch, 

 f. Physiol., Leipzig, 1877 to 1889 (Verhandl. d. physiol. Gesellsch. zu Berlin), republished 

 with notes in book form in 1890 ; and more recently in the Sitzunysb. d. k. Akad. d. 

 JVissensch. zu Berlin, 1892 and succeeding years. 



