THE CENTRAL NERVOUS SYSTEM 



857 



concerned in volitional movements is less decided than that ol 

 the precentral gyms, so its relation to afferent impulses, both 

 from the skin and the deeper structures, is better marked. From 

 the field of experiment further evidence of the sensori-motor 

 nature of the ' motor ' region is forthcoming. 



(i) 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 deli- 

 cate and skilled movements of the limb are either greatly impaired 

 or totally abolished 

 (Mott and Sherr ing- 

 ton). 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 electri- 

 cally or by inducing FIG. 363. DIAGRAM OF RELATIONS OF Occi 

 epileptic convulsions by 

 intravenous injection 

 of absinthe when 

 movements of the 

 affected limb take place 

 just as readily as move- 

 ments of the sound 

 limb. The cause of 

 the impairment of vol- 



FIG. 363. DIAGRAM OF RELATIONS OF 

 FITAL CORTEX TO THE RETINAE. 



RO, LO, right and left occipital cortex ; RE, 

 LE, right and left retina ; C, optic chiasma ; RF, 

 LF, right and left visual fields. The continuous 

 lines passing back from the retina? to the occipital 

 cortex represent the crossed, the broken lines the 

 uncrossed, fibres of the optic nerves and tracts. 

 For the sake of simplicity the intermediate stations 

 on the visual path in the anterior corpora quad 

 rigemina, lateral geniculate bodies, and pulvinar 

 are not represented in the diagram. For these 

 connections see Fig. 342, p. 819. 



untary 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 previously 



