ioo HUMAN PHYSIOLOGY 



applied for too long a period at one time, the acuteness of 

 perception diminishes. 



2. The Power of Localising the Place of Contact. 

 Where the tactile organs are abundant, the power of dis- 

 tinguishing accurately the point touched is more acute than 

 in places where these are more scattered. For this reason, if 

 two contacts are made at the same time, these may be very 

 close together in the former situation, and each of them may 

 be localised and felt as distinct from the other, whereas in 

 the latter situation, they would be felt as a single contact. 

 The power is therefore tested by determining how near two 

 points of contact may be brought to one another, and still 

 cause a double sensation. This may be done by means of 

 some form of aesthesiometer e.g. a pair of compasses ; and, in 

 using it, it is necessary to observe certain precautions. First, 

 the two points must touch the skin simultaneously. Second, 

 they must touch it lightly and with equal pressure each time. 

 Third, they must not be worked steadily from close together 

 to further apart, or vice versa, but must be used now in the 

 one way now in the other. (Experiment.) 



In this way it is easy to demonstrate that over the tips of 

 the fingers where tactile organs are very abundant the two 

 points of contact will give rise to a double sensation when 

 no more than 2 mm. apart, while over the back of the 

 hand they do not give a double sensation till they are about 

 30 mm. apart. Over the thigh and back they must be no 

 less than 50 to 70 mm. apart. 



Both the peripheral mechanism and the central nervous 

 system are involved in this localisation, and hence the power 

 varies with the condition of the skin as regards tempera- 

 ture, &c., and with the state of the nervous system. 



3. The Power of distinguishing Contacts in Time. If 

 the finger be brought against a toothed wheel rotated slowly, 

 the contacts of the individual teeth will be separately felt. 

 But if the wheel is made to rotate more and more rapidly, 

 the separate sensations are no longer felt, but a continuous 

 sense of contact is experienced. This indicates that, if stimuli 

 follow one another sufficiently rapidly, the sensations pro- 

 duced are fused. From this it is obvious that the sensation 

 lasts longer than the stimulus the contact. 



