X 



THE FOKE-BRAIN 



547 



body; with stronger currents the reaction spreads to more. 

 muscles, not only on the opposite, hut also ou the same side of the 

 hody. The mere displacement of the electrodes, or moving- them 

 away from each other, is enough to alter the form or extent of 

 the reaction. Lastly, if the electrodes are moved still further 

 from each other, or the current strengthened, epileptiform con- 

 vulsions set in which rapidly involve all the muscles. 



Hitzig and Fritsch gave the name of centres to those areas of 

 the cerebral cortex which, when excited with a weak current, 

 induce reaction in a limited group of muscles on the opposite 

 side. The position of these centres 

 is approximately constant in the 

 dog, taking into account the 

 different conformation of the sulci 

 in different races. They are 

 grouped round the sulcus cruci- 

 atus, which limits the so-called 

 sigmoid convolutions in the dog, 

 and also extend to the anterior 

 part of the second external con- 

 volution, as shown in Fig. 274. 



The excitable area of Fritsch 

 and Hitzig includes the centres for 

 the movements of the adductors, 

 flexors, and extensors of the limbs 

 on the opposite side, as well as the 

 centres which control the move- 

 ments of the face, head, and neck. 

 They evoked contractions of the 

 muscles of the back, tail, and 

 abdomen, on exciting points of 

 the brain surface lying between 

 those defined as centres, but were 

 unable to determine satisfactorily 

 any circumscribed point from which each of the above movements 

 could be separately excited. They stated that the whole of the 

 cerebral surface behind the centre for the facial muscles was 

 absolutely insensitive to the strongest electrical excitation. 



The galvanic current is not, however, the most appropriate 

 stimulus for the purpose for which it was employed by Hitzig 

 and Fritsch. Every closure or opening of the current produces 

 an electrolytic change in the cerebral surface at the points of 

 contact of the electrodes, which rapidly depresses and abolishes 

 excitability. This is not the case if faradic currents are employed, 

 and these can moreover be readily varied so as to adapt them to 

 the varying excitability of the motor points of the cerebral cortex. 



Ferrier (1873-1875), in determining the excitable points of the 



FIG. 274. Cortical motor centres of dog, 

 according to first experiments by Hitzig 

 and Fritsch. A, centre of neck muscles ; 

 + , of extensor and adductor muscles of 

 anterior limb; +, of flexors and rotators 

 of anterior limb; {{, of muscles of posterior 

 limb ; Q, of muscles of the face. The two 

 hemispheres belong to two different kinds 

 of dogs. 



