ANATOMICAL AND PSYCHIC LOCALISATION 267 



the tension of the extensors of the leg (and perhaps 

 also of other groups of muscles). For this reason 

 the leg slips easily and bends in the ankle-joint so that 

 the animal sometimes steps on the back instead of the 

 sole of the foot. It does not notice if the leg is 

 placed in an abnormal position. This is partly due to 

 the diminution in resistance caused by the weakening 

 of certain muscles and partly due to a reduction in the 

 sensibility of the skin. Goltz has proved that it re- 

 quires a greater pressure on the skin of this leg to 

 cause the dog to withdraw it than on any of the other 

 legs. This explains also why the dog does not notice 

 if the foot whose cortical centre has been removed is 

 placed in cold water. There are, then, changes in the 

 tension of certain muscles and a reduction in the 

 sensibility of the skin which suffice to explain all 

 the disturbances observed by Hitzig, but there is no 

 loss of muscular "consciousness" as Hitzig assumes. 

 To a certain extent, similar effects can be produced by 

 dividing the posterior roots of the arm-nerves. It 

 would hardly occur to anyone to maintain for this 

 reason that the psychic centre of the arm-movements 

 is localised in the posterior roots. Further proof that 

 these disturbances described by Hitzig are due to a 

 decrease in the tension of the extensors is furnished 

 by the fact that in man, when an arm becomes para- 

 lysed after a local disease in the cerebral hemispheres, 

 a contraction producing a flexed position of the arm 

 takes place after a time. Not all the muscles of the 

 arm are completely paralysed as a result of the disease 



