244 Animal Life and Intelligence. 



other words, the raison d'etre of the sense-organs is to set 

 agoing suitable activities — activities in due response to 

 the special stimuli. 



In this chapter we shall consider the modes in which 

 the special sense-organs are fitted to receive the influences 

 of the environment, deferring to a future chapter the con- 

 sideration of the resulting activities. For the present we 

 take these activities for granted, observing them only in so 

 far as they give us a clue to the sense-reaction by which 

 they are originated. In this chapter, too, we shall deal, 

 for the most part, with the physiological aspects of sensa- 

 tion. In all other organisms than ourselves, that is to say, 

 than each one of us individually for himself, the psycho- 

 logical accompaniments of the physiological reactions of 

 the sense-organs are matters of inference. Still, so closely 

 and intimately associated are the physiological and the 

 psychological aspects, that the exclusion of all reference to 

 the latter would be impracticable, or, if practicable, unad- 

 visable. What is practicable and advisable is to remember 

 that, even if the two are mentioned in a breath, the physio- 

 logical and the psychological belong to distinct orders of 

 being. 



In addition to the time-honoured "five senses," there 

 are certain organic sensations, so called, which take their 

 origin within the body. These are, for the most part, 

 somewhat vague and indefinite. They do not arise imme- 

 diately and in direct response to changes in the environ- 

 ment, but indicate conditions of the internal organs. Such 

 are hunger, thirst, nausea, fatigue, and various forms of 

 discomfort. Although they are of vital importance to the 

 organism, prompting it to perform certain actions or to 

 desist from others, they need not detain us here. 



More definite than these, but still of internal origin, is 

 the muscular sense. This, too, is of continual service to 

 every active animal. By it information is given as to the 

 energy of contraction of the muscles, and of the amount of 



