NEURO-MUSCULAR MECHANISM 



109 



2. Physiology 



The study of reflex action in the " spinal dog " (p. 93) has 

 revealed how various may be the reactions to different stimuli of 

 the skin, according to the part stimulated and to the character 

 of the stimulus. Thus harmful stimulation of the foot causes a 

 flexion and drawing up of the leg, pressure of the sole with 

 the finger causes the extensor thrust. Again, gentle stimula- 

 tion on the shoulder causes the characteristic scratching reflex. 



Nocuous stimuli seem to cause a different reflex result from 

 non-nocuous. They are antagonistic, and the nocuous stimulus 

 is generally prepotent, and is able 

 to replace in reflex action the 

 non-nocuous. The sensation 

 which accompanies their stimula- 

 tion is not that of contact but of 

 pain, and the central neurons 

 which transmit such impulses run 

 independently of those transmit- 

 ting true touch impulses. 



The tactile sense may be best 

 studied under three heads : 



1. The Power of Distinguish- 

 ing Differences of Pressure. 

 Variations of pressure in time 

 and space are alone distinguished. 

 We live under an atmospheric 

 pressure of 760 mm. of mercury, 



. but this gives rise to no sensa- 

 tion. Any sudden increase or diminution of pressure, however, 

 leads to a marked change of sensation, but a slow change causes 

 a lesser modification of consciousness. 



2. The Power of Localising the Place of Contact. 

 Where the tactile organs are abundant, as in the lips of the 

 horse, the power of distinguishing accurately the point touched 

 is more acute than in places where these are more scattered. 



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. 



FIG. 49. Simple form of sensory 

 nerve termination. In the 

 tactile corpuscle the nerve fibre 

 coils round the capsule before 

 entering. (DooiEL.) 



