102 



VETERINARY PHYSIOLOGY 



of the meclianisin, peripheral or central, (a) Peripheral — 

 When the skin is cold, the sense is much less acute than 

 when it is warm, (b) Central — The nerve centres are readily 

 fatigued, and the power of appreciating sensation is thus 

 decreased. 



2. The Power of Localising the Place of Contact. — Where 

 the tactile organs are abundant, the power of distinguishing 



accurately the point touched 

 i^^ k^ V^ 1/^ k^ is more acute than in places 



where thev are more scat- 

 tered. For this reason, if two 

 contacts are made at the same 

 time, they may be very close 



r-ktvK-h 



Fig. 44. — Relationship of Sensa- 

 tion to Stimulus, with weak and 

 strong stimuli. Stimuli repre- 

 sented by vertical lines — the 

 strength being indicated by 

 their height. Sensations repre- 

 sented by the curves. 



together in the former situation, 

 e.g. the lips, and each of them 

 may be localised and felt as 

 distinct from the other, whereas 

 in the latter situation they may 

 be felt as a sinsrle contact. 

 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 felt separately. 

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

 the separate sensations are no longer felt, and a continuous 

 sense of contact is experienced (Practical Physiology). This 

 indicates that, if stimuli follow one another sufBciently rapidly, 

 the sensations produced are fused. From this it is obvious 

 that the sensation lasts longer than the stimulus — the 

 contact (fig. 44), 



The duration of the sensation depends upon the degree 

 of stimulation of the peripheral tactile organs. 



Probably two sets of receptors are involved in the tactile 

 sense. 



1st. The tactile corpuscles, which are stimulated by direct 

 pressure on the surface of the skin. It is these which form the 

 centres of the touch spots already mentioned. They are best 

 investigated by von Frey's bristles — bristles of varying thick- 

 ness fixed in suitable handles and with the pressure required 

 to bend them determined by pressing on a spring balance. 



