256 ORIGINAL ARTICLES AND CLINICAL CASES 



leave undecided the question whether this is their normal function or 

 merely a vicarious function due to the destruction of the stimulable 

 cortex. In a previous study of the role of various parts of the cerebrum 

 of the white rat in the formation and retention of visual habits the 

 writer found that the habit of discrimination of light intensities survived 

 the total destruction of the stimulable area. Injury to an area on each 

 occipital lobe corresponding to Brodmann's area striata [2] resulted 

 in the complete loss of the visual habit, but any other third of the 

 cortex might be destroyed without in the least disturbing the animal's 

 ability to ma.ke the discrimination accurately. The data reported in 

 that study seem to prove that the conditioned-reflex arcs involved in 

 visual habits descend from approximately the same area of the cerebral 

 cortex to which they are projected and do not traverse the cortex to 

 descend from the stimulable areas. The visual habits have cerebral 

 representation, but the supposed motor areas have no direct function 

 in their performance [10]. 



In higher animals, especially the primates, complete paralysis of 

 voluntary movement may follow destruction of the stimulable cortex, 

 but this does not necessarily imply an interruption of the direct 

 conditioned-reflex arcs. Such an explanation is to be preferred on 

 grounds of simplicity, if contradictory facts do not appear, bat dis- 

 turbances of some of the mechanisms of tonic reinforcement, not in the 

 direct conditioned-reflex path (section of the afferent nerves of a limb, 

 cerebellar injury, &c), may produce somewhat similar disturbances of 

 voluntary movement and indicate an alternative explanation of the 

 function of the stimulable areas. The demonstration that, in the rat, 

 the stimulable area is not traversed by the conditioned-reflex arcs of 

 visual habits seems sufficient evidence to raise the question of whether 

 the so-called motor area is really a part of the mechanism for voluntary 

 movement or is merely a part of the subsidiary tonic and postural 

 mechanism. 



Generalization from the rat to higher forms is complicated, how- 

 ever, by the probable transfer of function from the corpus striatum to 

 the stimulable cortex, with ascent in the evolutionary scale. In the 

 rat, destruction of the stimulable areas produces no detectable motor 

 disturbances, but simultaneous destruction of the motor area and the 

 corpus striatum produces a paralysis which is relatively permanent and 

 which resembles the effect of destruction of the stimulable area in 

 higher forms [10]. This indicates that the function of the stimulable 

 area of primates is represented in the rat by the combined function of 



