The Senses of Animals. 245 



movement effected — not to mention the rapidity and dura- 

 tion of the muscular effort. By it the position, or changes 

 of position, of the motor-organs are indicated. It is 

 obvious, therefore, that the sensations obtained in this way, 

 some of which are exceedingly delicate, are an important 

 guide to the organism in the putting forth of its activities. 

 It is through the muscular sense that we maintain an 

 upright position. It is through an educated and refined 

 muscular sense that the juggler and the acrobat can 

 perform their often surprising feats. Concerning the 

 physiology of the muscular sense, we have at present no 

 very definite knowledge. Some have held that we judge of 

 muscular movements by the amount of effort required to 

 initiate them ; but it is much more probable that there are 

 special sensory nerves, whose terminations are either in 

 the muscles themselves or in the membranes which sur- 

 round them. 



We come now to the special senses. Of these we will 

 take first the sense of touch. Through this sense we are 

 made aware of bodies solid or liquid (or perhaps gaseous) 

 which are actually in contact with the skin or its infold- 

 ings at the mouth, nostrils, etc. There are considerable 

 differences in the sensitiveness of the skin in different parts 

 of its surface ; some parts, like the filmy membrane which 

 covers the eye, being very sensitive, while others, like the 

 horny skin that covers the heel of a man who is accustomed 

 to much walking, are relatively callous. Different from 

 this is the delicacy of the sense of touch. This delicacy is 

 really the power of discrimination, and therefore involves 

 some mental activity. But it is also dependent upon the 

 distribution of the recipient end-organs of the nerve. The 

 highest pitch of delicacy is reached in the tip of the tongue, 

 which is about sixty times as delicate as the skin of the 

 back. The power of discrimination is tested in the follow- 

 ing way : The points of a pair of compasses are blunted, 

 and with them the skin is lightly touched. When the 

 points are close together, the sensation is of one object; 



