RECEPTOR MECHANISMS CUTANEOUS SENSATIONS 591 



of sensation; the surface and deep sense organs may be compared to 

 a pair of thermo-electric junctions. The adequate stimulus depends 

 not only on the intensity of the stimulus, but upon the actual size 

 of the region stimulated. It is probably for this reason that water 

 at 37 C. feels warmer to the whole hand immersed in it than does 

 water at 40 C. to one finger only. The degree of sensation evoked 

 also depends upon the thermal capacity and conductivity of the body 

 applied to the skin. 



The parts of the body in which the thermal sense is most acute 

 are the tip of the tongue, the eyelids, cheeks, lips, and belly. The 

 laundress tests the warmth of her iron with her cheek, and not with 

 her hand, and the bared elbow is used by a good nurse to test the 

 temperature of a hot bath. The temperature sensations regulate to 

 a large extent the tone of the skeletal muscles, the vaso-motor tone, 

 the activity and metabolism of the body. The play of wind, sunlight 

 and shadow stimulate the nervous system and prevent monotony. 



The Sensation of Pain. It seems probable that special " pain 

 spots " exist for the appreciation of pain. They have a long latent 

 period when subjected to weak stimulation, and do not react easily 

 to rapidly alternating or oscillating stimuli. They do not coincide 

 with pressure-points, and are about four times as numerous as these. 

 It has been suggested that the free nerve-endings in the skin are the 

 special organs excited. In support of this view, it has been 

 shown 



1 . That diminution of the surface area stimulated does not diminish 

 the effectiveness of the stimulus. 



2. That the electrical stimulus is more effective than any other 

 form. 



3. That the first sensation produced by the application of a corrosive 

 is one of pain. 



It is stated also that from the cornea only sensations of pain arise, 

 and that here the only receptive elements are the intra-epithelial 

 nerve -endings. 



It is probable, however, that pain may arise from excessive stimu- 

 lation of the organs connected with the other sensations of the skin. 

 Excessive heat or cold produces the same kind of sensation of pain. 



Frotopathic and Epicritic Sensibility. A careful study of the sensory 

 changes associated with the experimental division and regeneration of 

 cutaneous nerves in trained observers, has led to a new classification 

 of the sensibilities of the skin. In the area supplied by the severed 

 nerves the sensations of heat, pain, and cold, were lost, also sensa- 

 tion of touch to cotton-wool, and to the pulling of the skin outwards. 

 Pressure inwards was appreciated and well localized that is, deep 

 pressure due to sensation in the underlying muscles. After seven 

 weeks, sensibility to pin-prick returned, but it required a higher 

 minimal stimulus than the normal parts, and produced a peculiar 

 unpleasant sensation which radiated and tended to be localized in 

 remote parts. Water of 38 to 50 C. was recognized as hot, and 



