THE FOEE-BEAIN 



549 



different experimental conditions to which the animal is exposed 

 (derive of narcosis, haemorrhage during the operation, hyperaemia 

 or isrliat'inia of the cortex), but the excitability of Hie different 

 centres of the same animal also varies, as well as the extent of 

 the areas which each occupies. This is shown diagram matically 

 in Fig. 276. 



A new fact which we discovered in 18*78 is that the motor 

 centres for the limbs of the 

 dog are not limited to the 

 surface of the postcruciate part 

 of the sigmoid gyrus, but ex- 

 tend into the portion of the 

 cortex that dips into the sulcus, 

 which we found to be about 

 three times as extensive as the 

 excitable area on the surface. 

 When an induced current is 

 applied by suitably protected 

 electrodes, reactions of the 

 hind-limb on the opposite side 

 arc obtained when the elec- 

 trodes are placed on the most 

 internal and median part of 

 the introtiected cortex; and re- 

 actions < if the opposite fore- limb 

 (in exciting the outer part of 

 the cortex. 



Later on (1883) we found 

 that the cortex within the 

 sulcus cruciatus of the dog is 



excitable, not merely tO faradic FIG. 276. Asymmetrical localisation of the motor 

 ! . . i i i centres in the postcruciate 



stimulation, but also to mechani- 

 cal stimuli. To demonstrate this 

 it is necessary to divide the 

 arachnoid that unites the two 

 edges of the cruciate sulcus, 

 avoiding the vein that passes 



through it, and to introduce a metal probe with sharp edges 

 carefully through the opening, and pass it along the sulcus so 

 as to scrape the introflected cortical surface. The usual com- 

 plex motor reactions of the muscles of the limbs on the opposite 

 side will be at once obtained; those of the posterior limbs on 

 scraping the inner and deeper part, and of the anterior limbs 

 on scraping the outer and superficial part of the introflected 

 cortex. The reactions do not differ from those obtained with 

 electrical stimulation, but they are usually less vigorous, and 

 after being once elicited, do not recur on repeating the stimulus, 



postcruciate part of clod's 

 sigmoid gyrus. (Lnciani and Tainlmrini.) a, 

 abduction and flexion of posterior limb of 

 opposite side ; a', elevation and advance of same 

 limb; 6, abduction and elevation of oppn^iti' 

 fore-limb; V, flexion of forearm on arm with 

 movement of opposite shoulder ; b", retraction 

 and adduction of opposite fore-limb; c, move- 

 ments of head and neck. 



