1911.] the Transmission of Acquired Characters. 



569 



this side, but in other cases later in the experiment they appear upon the 

 opposite side. 



The section of the great sciatic nerve causes two kinds of change in the 

 individual. Of these the first is that peripheral to the place of section of 

 the nerve. There is paralysis of certain of the muscles of the lower limb 

 (extensors and flexors of the ankle and the peroneal muscles and the short 

 muscles of the foot), and ansesthesia of a certain peripheral field which 

 indirectly produces a loss of the toes. The second change is central. The 

 excitability of the scratch-reflex is raised, and at the same time there is 

 a parallel change in the peripheral field of skin in the neck and over the face. 



Concerning the causation of the Brown-S^quard phenomenon, it might be 

 argued either, on the one hand, that the raised excitability which conditions 

 it is caused by over-stimulation of the afferent part of the mechanism by the 

 degenerative changes in the skin area of the neck, and that these are due to 

 the absence of proper grooming in virtue of the paralysis of the leg and foot 

 and the loss of the scratching claws; or, on the other hand, it might be 

 supposed that the section of the nerve produces a central change, one feature 

 of which is a raised excitability of the scratch-reflex, and that the degenera- 

 tive changes in the skin of the neck are conditioned either directly or 

 indirectly by this central change and not by the peripheral paralysis and 

 loss of toes. 



The first theory is probably not correct. The Brown-Sdquard phenomenon 

 may be present when there is no visible degenerative change in the skin area 

 of the neck and face. It may also be present when the scratching toes are 

 not lost, and when the " voluntary " scratching movements are actually 

 greater than usual upon that side. " Voluntary " scratching may even be so 

 exaggerated and so effectual as to produce a bare patch of skin such as that 

 mentioned above. 



The second view must be held. Section of the great sciatic nerve actually 

 raises the excitability of the scratch-reflex, and the degenerative changes in 

 the neck area are due, not to excess of grooming, but to a diminution of 

 grooming over the greater part of the area. This seeming paradox may be 

 explained as follows :— The scratch-reflex varies as it is evoked from 

 different areas of the total receptive field. Each of these various reactions 

 is really, as Sherrington points out (44), an individual scratch-reflex different 

 from all others. The section of the great sciatic nerve may be supposed to 

 raise the reflex excitability for only a small group of these. That this is so 

 is demonstrated by the restricted area from which the Brown-Sequard 

 reaction may at first be obtained (45). When a " normal " stimulus — tickling 

 caused by lice — occurs in the neck area, the scratch-reflex will be such that 



