344 SPECIAL PHYSIOLOGY. 



ences in kind, or in degree, in regard to each sense ; the peculiarities 

 or different degrees of these changes may depend on the difference in 

 the external stimuli which, in the case of each special sense, are able 

 to act upon the nervous substance: and lastly, the object of the spe- 

 cialized recipient surface or apparatus in each sensory organ, the only 

 part of each organ of sense in which we can detect most manifest, varied, 

 and singularly adapted structure and contrivance, may be to mediate 

 between the special stimulus and the common fundamental nervous en- 

 dowment, to translate that stimulus into nervous energy, and so to ex- 

 cite peculiar modes of reaction in a similar nervous substance. When 

 our knowledge concerning the conditions or changes which take place 

 in the nervous substance in common sensation is complete, we may be 

 able to explain the modifications in those changes or conditions which 

 are essential to special sensation. 



All sensations, though realized through the mental state called con- 

 sciousness, which is beyond our means of investigation, depend ultimately, 

 as objects of physiological study, on certain corporeal states or changes 

 in the sensorial nervous centres. These changes or states are produced, 

 as we have explained, either by external or internal stimuli ; the exter- 

 nal stimuli in question differ remarkably in their source and nature ; 

 and there must obviously be a strict correspondence between the variety 

 of reactions in the nervous substance, and the nature of these stimuli. 

 May not, therefore, the differences presented by the various stimuli 

 to sensation furnish us with the means of classifying the sensations 

 themselves ? Thus the external stimuli produce distinct impressions cor- 

 responding to the sensations of sight, hearing, smell, taste, the tactile 

 sense, and the sense of temperature, all of which may be regarded as 

 distinct senses. Sight informs us of the existence of light and color ; 

 the sense of temperature conveys to us the knowledge of the existence 

 of heat ; and these two senses would seem to have a certain natural al- 

 liance with each other. Again, the senses of smell and taste are also 

 allied. Lastly, the sense of hearing, as we shall immediately show, is 

 closely allied to that of touch. All the senses are excitable by the 

 electric energy. 



These six special senses may, according to this view, be arranged 

 in three groups, each containing two related or coupled senses. The 

 first group consists of two molar or dynamical senses, viz., touch and 

 hearing, the senses of matter in contact and of matter in motion, or the 

 tactile and acoustic senses ; the former reveals to us the presence of 

 matter itself by the pressure of substance against substance, whilst the 

 latter conveys to us the effects produced by particles of matter under- 

 going motions which cause the phenomenon of sound. The second 

 group is chemical, and includes taste and smell, the former acting dia- 

 lytically, the latter perhaps catalytically. Both depend on chemical 

 reactions, which take place in the extremities of their respective nerves, 

 and are allied by the common property of recognizing those forms of 

 molecular motion which occur in acts of chemical combination or de- 

 composition. Taste, however, requires as one of its conditions the 

 dialytic penetration of a chemical substance in solution, through the 

 soft tissues to the extremities of the gustatory nerves ; whilst smell 



