568 Mr. Graham Brown. Alleged Specific Instance of [Dec. 9, 



In this area of the skin the hair sometimes, but not always, becomes 

 rough and broken and is shed. At the same time lice accumulate in the 

 area and the skin is thickened. 



In the case of one guinea-pig a very instructive observation was made. 

 This animal exhibited a bare crescentic patch of skin behind the scapula 

 of that side upon which the Brown- Sequard phenomenon might be elicited 

 in consequence of the removal of part of the great sciatic nerve. This 

 was certainly caused by excessive " voluntary " scratching directed to this 

 very limited area. From this it was argued that the normal scratching 

 movements were especially frequent upon the side of the lesion in 

 guinea-pigs exhibiting the phenomenon. This supposition also rests upon 

 direct observation. From this and other considerations it was concluded 

 that the roughening of the hair (and the other changes in the special area of 

 skin) are not caused directly by the section of the great sciatic nerve — either 

 by an effect in producing a greater amount of scratching than is usual, or by 

 an effect in producing a less amount of effective scratching in consequence of 

 loss of the toes by the trophic changes in the foot — but that it might either 

 follow indirectly a central change in the nervous system, or, what is more 

 probable, be due indirectly to a lessened amount of effective scratching. 

 This last supposition may be explained by supposing that the effect of the 

 section of the nerve is to cause an increase in the excitability of the scratch- 

 reflex evoked in response to the mechanical stimulation of a circumscribed 

 part of the special area. When a normal stimulus falls outside this the 

 scratch is at first directed to the stimulated point — is co-ordinate — but almost 

 immediately irradiates to the point which is especially excitable. Thus all 

 other areas receive inadequate grooming although the scratch-reflex itself 

 may be described as relatively excitable. 



The experiments described in a previous section (III, (b) ) demonstrate 

 not only that the " trophic " change in the foot is not the cause of the raised 

 excitability seen in the condition, but that the possibility of effective or 

 ineffective scratching (as regards only the state of the scratching implement) 

 is not a determining cause. 



That the scratch-reflex is especially excitable upon the side of which the 

 great sciatic nerve has been divided is shewn not only by the occurrence of 

 the Brown-S^quard phenomenon itself but also by evidence derived from the 

 phenomena of the " narcosis scratch " as it appears in such individuals (47). 



In these cases the " narcosis scratch " does not run its ordinary 

 symmetrical course — alternating from side to side of the body. More 

 usually it occurs at first only upon the same side as that which exhibits the 

 Br^wn-Sequard phenomenon. The movements may be confined entirely to- 



