85 8 A MANUAL OF PHYSIOLOGY 



mentioned (p. 790), 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 par- 

 tially 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, 

 contraction 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 racliata. 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 stimula- 

 tion and contraction is greater by nearly 50 per cent, when the 

 cortex is stimulated than when the white fibres are directly excited. 

 (b) Morphine greatly increases the period of delay for stimulation of 

 the cortex, and at the same time renders the resulting contractions 

 more prolonged than normal, while the results of direct stimulation 

 of the white fibres are much less, if at all, affected, (c) Stimula- 

 tion of the grey matter, when separated from the subjacent 

 white matter by the knife, but left in position, is without 

 effect unless the strength of stimulus be increased, although twigs 

 of the current ought, of course, to pass into the corona radiata as 

 easily as before. Perfectly definite movements can, however, be 

 excited or inhibited by stimulating definite spots in the corona 

 radiata, and even in the internal capsule. This simply means that 

 in these positions the fibres representing these movements are not 

 yet intermingled with fibres representing other movements. 



Sensory Areas Visual Centres. In the occipital lobe in 

 animals an area of considerable extent has been found, destruc- 

 tion of which causes hemianopia (p. 819). Thus, if the right 

 occipital cortex is destroyed, the right halves of the two retinae 

 are paralyzed, and the left half of the field of vision is a blank. 

 There is conjugate deviation of the head and eyes to the same side 

 as the lesion in other words, the animal turns its head and eyes 

 to the right. Destruction of this region on both sides causes 

 complete blindness. When the same region is stimulated, the 



