THE FOKE-BBAIN 



549 



different experimental conditions to which the animal is exposed 

 (degree 'of narcosis, haemorrhage during the operation, hyperaemia 

 or ischaemia of the cortex), but the excitability of the different 

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

 the areas which each occupies. This is shown diagrammatically 

 in Fig. 276. 



A new fact which we discovered in 1878 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 

 are obtained when the elec- 

 trodes are placed on the most 

 internal and median part of 

 the introflected cortex ; and re- 

 actions of the opposite fore-limb 

 on 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 



, T , . -, i i , i centres in the postcruciate part of dog's 



Stimulation, but also tO mechani- sigmoid gyrus. (Luciani and Tamburini.) a, 



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, 



abduction and flexion of posterior lirnb of 

 opposite side ; a', elevation and advance of same 

 limb ; b, abduction and elevation of opposite 

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

 movement of opposite shoulder ; h", retraction 

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

 ments 'of head and neck. 



