754 ^ MANUAL OF PHYSIOLOGY 



tion set up by localized disease may spread far and wide from its. 

 original focus, involving area after area of the Rolandic region first 

 in the one hemisphere and then in the other. The part of the body 

 to which a sensory aura is referred is as significant an indication of 

 the seat of the discharging lesion as is the part of the body which, 

 first begins to twitch. 



Sensory Functions of the Rolandic Area. This is one of the 

 proofs that the Rolandic region is not a purely motor, but a sensori- 

 motor, or kittastheiic, area. Histological studies on the course of 

 the sensory paths support this conclusion. We have already mentioned 

 that, according to Goltz's observations (p. 747), removal of the 

 Rolandic cortex causes defects of sensation as well as of movement. 

 From the field of experiment further evidence is forthcoming. 



(1) It has been found that if the posterior roots of the nerves 

 supplying one of the limbs be cut in a monkey, all the most delicate 

 and skilled movements of the limb are either greatly impaired or 

 totally abolished (Mott and Sherrington). The limb is not used for 

 progression or for climbing, but hangs limp, and apparently helpless, 

 by the side of the animal. That this condition is not due to any 

 loss of functional power by the peripheral portion of the motor path 

 may be assumed, since the anterior roots remain intact. That it is 

 not due to any want of capacity on the part of the motor centres to 

 discharge impulses when stimulated may be shown by exciting the 

 cortical area of the limb either electrically or by inducing epileptic 

 convulsions by intravenous injection of absinthe when movements 

 of the affected limb take place just as readily as movements of the 

 sound limbs. The cause of the impairment of voluntary motion, 

 then, can only be the loss of the afferent impulses which normally 

 pass up to the brain, and presumably to the motor cortex. When 

 only one sensory nerve-root is cut, no defect of movement can be 

 seen ; and this is evidently in accordance with the fact already men- 

 tioned (p. 698), that complete anaesthesia of even the smallest patch 

 of skin is never caused by section of a single posterior root. And 

 that it is the loss of impulses from the skin which plays the chief 

 part is shown by the fact that after division of the posterior roots 

 supplying the muscles of the hand or foot, which only partially inter- 

 feres with the sensory supply of the skin, joints, sheaths of tendons, 

 etc., movement is unimpaired ; while section of the nerve-roots sup- 

 plying the skin, those of the muscles being left intact, causes extreme 

 loss of motor power. 



(2) If a strength of stimulus be sought which will just fail to cause 

 contraction of the muscular group related to a given motor area, and 

 a sensory nerve, or, better, a sensory surface (best of all, the skin 

 over the corresponding muscles), be now stimulated, contraction will 

 occur that is to say, the excitability of the motor centres will be 

 increased. This shows that the motor region is en rapport not only 

 with efferent, but also with afferent fibres, that it receives impulses as 

 well as discharges them. 



The same experiment is a proof that the results of excitation of 

 the motor cortex are due to stimulation of the grey matter, and not, 



