268 THE SPECIAL SENSES. 



ferent qualities which we designate by special names, — the series 

 of different colors, for example. In sound sensations we distinguish 

 different tones and different qualities of tones. But here, again, 

 the subjective mark is often so indistinct in consciousness 

 that it cannot be used satisfactorily for purposes of classifica- 

 tion. In the odor sensations we distinguish many different quali- 

 ties, each recognizable at the time that it is experienced, but their 

 characteristics are so fugitive that so far it has not been possible 

 to name them or group them in any satisfactory way. In studying 

 the quaUties of the various sensations, so far as they are recogniz- 

 able, the effort of physiology has been to connect them with some 

 definite anatomical or physiological peculiarity in the sense organs 

 concerned. The final explanation of the differences in quaUty 

 involves a study of the nature and properties of consciousness 

 itself, — a subject which as yet has not been undertaken by physi- 

 ology. At present we accept the fact of consciousness and the 

 fact that there are different kinds or qualities of consciousness, 

 and our investigations are directed only toward ascertaining 

 the anatomical, physical, and chemical properties of the organs 

 involved in the production of these subjective changes. 



In former times it was customary to divide the sensations into 

 two different groups, — the special and the common senses, — the 

 former including the so-called five senses of man, — namely, sight, 

 hearing, touch, taste, and smell, — while under the latter were 

 grouped all other sensations of less distinctive qualities. In physi- 

 ology the belief that man has only five special senses has, however, 

 long been abandoned. The sense of touch as ordinarily understood 

 has been shown to consist of three or more distinct senses, namely, 

 pressure (in its several varieties), heat and cold; and the sense 

 of pain exhibited by the skin is in all essential respects as special 

 and characteristic as those just named. There is, however, no 

 certain standard as to what shall constitute a special in con- 

 tradistinction to a common sense; so that a classification based 

 on this nomenclature is unsatisfactory. In one respect, how- 

 ever, our senses show a difference which may be used as a basis 

 for dividing them into two general groups. This difference 

 lies in the manner of projection. We may assume that all of 

 our sensations are aroused directly in the brain. In that organ 

 take place the final changes which react in consciousness. But 

 in no case are we conscious that this is the case. On the 

 contrary, we project our sensations either to the exterior of 

 the body or to some peripheral organ in the body, the effort 

 being apparently to project them to the place where experi- 

 ence has taught us that the acting stimulus arises. We may 

 divide the senses, therefore, into two great groups: (1) The 



