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EXPERIMENTAL PHYSIOLOGY. 



(b.) Use a small pair of wooden compasses, or an ordinary 

 pair of dividers with their points guarded by a small piece 

 of cork, or Sieveking's JSsthesiometer. Apply lightly the 

 points of the compasses simultaneously to different parts of 

 the body, and ascertain at what distance apart the points are 

 felt as two. The following is the order of sensibility : Tip 

 of tongue (1-1 mm.), tip of the middle finger (2'3), palm 

 (8 to 9), forehead (22), back of hand (31-6), back (66). 



(c.) Test as in (b.) the skin of the arm, beginning at the 

 shoulder and passing downwards. Observe that the sensi- 

 bility is greater as one tests towards the fingers, and also in 

 the transverse than in the long axis of the limb. 



(d.) By means of a spray-producer spray the back of the 

 hand with ether, and observe how the sensibility is 

 abolished. 



(e.) In all cases compare the results obtained on both 

 sides of the body. 



(/) Illusions. Aristotle's Experiment. Cross the middle 

 over the index finger, as in Fig. 142, roll 

 a small ball between the fingers ; one has a 

 distinct impression of two balls. Or, 

 cross the fingers in the same way, and 

 rub them against the point of the nose. 

 The same illusion is experienced. 



2. The Sense of Temperature. 



(a.) Ask the person experimented on 

 to close his eyes. Use two test-tubes, 

 one filled with cold and the other with 

 hot water; or two spoons, one hot and 

 Fig. 142. one cold. Apply one or other to different 



parts of the surface, and ask the person 

 to say whether the touching body is hot or cold. Test 

 roughly the sensibility of different parts of the body with 

 cold and warm metallic-pointed rods. 



(b.) Touch a ball of fur, wood, and metal. Observe that 

 the metal feels coldest, although all the objects have the same 

 temperature. 



