196 EXPERIMENTAL PHYSIOLOGY. 



bed iscriminated (touch discrimination). This "local sign", as it 



is called, varies in different parts of the body. 



Experiment 88. With a pair of calipers, determine this distance 

 for the tip of the tongue, the palmar surface of the third phalanx 

 of the finger, its dorsal surface, the tip of the nose, the back and 

 upper arm; make a table of the results. Note that the order 

 of sensitivity for touch discrimination is not the same as that 

 for touch acuity. 



SENSE OF PAIN. 



Any stimulus will cause pain when it is very intense. This does 

 not mean, as has been thought, that pain is merely due to over- 

 stimulation of any receptors. There are special receptors for pain, 

 just as there are for touch and temperature. One of the main 

 proofs of this is that pain spots are more extensively and regularly 

 distributed than the others. 



Experiment 89. Using the same area of skin as was employed 

 for the location of temperature and touch spots, proceed to mark 

 out the pain spots by pricking with a stout horse hair or a bristle. 

 Note that the sensation is more lasting than that for touch and 

 that the pain spots are distributed in a different manner. 

 Another proof of the separate existence of pain spots is that they 

 alone are found in the cornea; on the conjunctiva they exist along 

 with temperature spots. 



Experiment 90. Verify the above statement, using medium 

 hairs. 



The skin sensations are not at all influenced to the same ex- 

 tent when the blood supply is interfered with. This fact becomes 

 of interest on account of its analogy to the results which have 

 been observed during the regeneration of severed nerves. 

 Experiment 91. By means of a blood pressure cuff and bulb 

 applied to the arm, determine the systolic pressure, then release 

 the pressure and raise the arm so as to empty it of some of its 

 blood, after which again apply the pressure (but not above the 

 systolic value). Proceed to compare the sensations of touch, 

 temperature and pain in exactly corresponding skin areas of the 

 two hands. Differences may take some time to become evident. 

 Note particularly for which sensations they first become so. 



