Ch.p. TIL] or SENSE ORGANS. 67 



which have heen clearly pointed out by Herbert Spencer. 

 *' At the surface of the body," he says,* " where the ex- 

 tremities of nerve fibres are so placed as to be most easily 

 disturbed, we generally find what may be called multipliers 

 of disturbances. Sundry appliances, which appearing to 

 have nothing in common, have the common function of 

 concentrating, on the ends of nerves, the actions of ex- 

 ternal agents." This effect is produced by lenses in the 

 eyes, otoliths and other bodies in the organs of hearing, 

 vibrissas and corpuscida tactus in the skin ; all of which 

 serve to exaggerate the effects of incident forces upon 

 especially sensitive peripheral expansions of the nervous 

 system. " The ultimate nerve fibrillse, ramifying where 

 they are most exposed to disturbances, consist of nerve 

 protoplasm, unprotected by medullary sheaths, and not 

 even covered by membranous sheaths. In fact they 

 appear to consist of matter like that contained in nerve 

 vesicles, .... and may be regarded as, like it, more 

 unstable than the matter composing the central fibres of 

 the fully difterentiated nerve tubes. . . . This peri- 

 pheral expansion of the nerve on which visual images fall 

 contains numerous small portions of the highly unstable 

 nerve matter, ready to change, and ready to give out 

 molecular motion in changing. It is thus, too [in higher 

 animals], with those terminal ramifications of the auditory 

 nerve on which sonorous vibrations are concentrated. 

 And there is an analogous peculiarity in the immensely 

 expanded extremity of the olfactory nerve. Here, over a 

 large tract covered by mucous membrane, is a thick plexus 

 of the grey unsheathed fibres ; and among them are 

 distributed both nerve vesicles and granular grey sub- 

 stance, such as that out of which the vesicles arise in 

 the nervous centres." 



* " Principles of Psychology," vol. i p. 35. 



