570 Mr. Graham Brown. Alleged Specific Instance of [Dec. 9; 



the scratching toes are directed to the point stimulated, and the reaction is 

 an effective grooming of the skin. But if the excitability of a neighbouring 

 group of arcs is raised by section of the sciatic nerve, then the normal 

 response thus engendered may rapidly become one directed by irradiation 

 towards the most excitable area. There will, in consequence, be a diminished 

 normal grooming of the skin in all other areas. This will lead to an 

 accumulation of lice, etc., and this condition may help to augment the raised 

 excitability of the scratch-reflex. 



But in what manner may the section of the great sciatic nerve produce the 

 primary raised excitability of the scratch-reflex ? 



The author has previously described a conception of " neural balance " 

 (59, 60). In general, it may be supposed that the activity and the reflex 

 excitability of a centre — or of a pair of antagonistic centres — go hand in 

 hand, and that they are the resultant of innumerable excitatory and inhibitory 

 influences which continually play upon the centre. This resultant may be 

 termed the " neural balance." The neural balance may be tilted for a time 

 in one direction by the temporary preppnderance of one of the influences 

 which play upon it. And it may be permanently upset by a permanent 

 alteration in the value of the influences which compose the balance. 



With regard to the scratch-reflex, it has already been shewn (53) that 

 removal of the cerebral cortex of one side produces such a permanent 

 alteration. It is probable that section of the great sciatic nerve does also. 



That the removal of so great a sensory field as that subtended by the 

 sciatic nerve should increase reflex excitability in certain definite directions 

 is likely. It is by no means improbable that the activity of the afferent 

 mechanisms contained therein exerts an inhibitory influence upon the 

 scratch-reflex ; and that the section of the nerve permanently removes this 

 inhibition. 



In this connection it is interesting to refer to the observations of some 

 other investigators. For instance, Langendorff (30) found that Goltz's 

 croak-reflex — which may be evoked in the frog with regularity after 

 decerebration — may also be evocable thus after section of the optic tracts 

 alone. The removal of a peripheral sensory field here makes a specific 

 reflex more excitable. He points out, too, that the raised excitability of 

 the hind limbs, which is produced by packing the back of a frog in ice 

 and was observed by Freusberg (28), may be due to the abolition in this 

 manner of a large sensory field in the skin. 



Of particular interest are the observations of v. Botticher (31). He 

 repeated Langendorffs experiments, and found, in addition, that section of 

 both great sciatic nerves admitted of the regular evocation of the croak- 



