93 THE CENTRAL NERVOUS SYSTEM 



deeper structures, is better marked. From the field of experiment 

 further evidence of the sensori-motor nature of the ' motor ' region 

 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 help- 

 less, 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 limt 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 limb. 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 move- 

 ment can be seen ; and this is evidently in accordance with the fact 

 previously mentioned (p. 863), 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 

 interferes with the sensory supply of the skin, joints, sheaths of 

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

 roots supplying 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, con- 

 traction may occur that is to say, the excitability of the motor 

 centres may 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, as 

 might be objected, of the white fibres of the corona radiata. It is 

 undoubtedly possible to excite these fibres by electrodes directly 

 applied to the motor cortex, but in the latter case the current has to 

 be made stronger than is sufficient to excite the grey matter alone. 

 Further evidence is afforded by the following facts : (a) The ' period 

 of delay ' that is, the period which elapses between stimulation and 

 contraction is greater by nearly 50 per cent, when the cortex is stimu- 



