368 PKACTICAL PHYSIOLOGY. [LXXVI. 



(i.i 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 (6.) the skin of the arm, beginning at the shoulder 

 and passing downwards. Observe that the sensibility is greater as 

 one tests towards the fingers, and also in the transverse than in the 

 long axis of the limb. In all cases compare the results obtained on 

 both sides of the body. 



(d.} By means of a spray-producer spray the back of the hand with ether, 

 and observe how the sensibility is abolished. 



(c.) V. Frey's Method. A "hair of the head or beard (20-40 mm. long) is 

 fixed to a wooden match. On pressing the point of the hair against the skin 

 it may or may not be felt as a tactile sensation. This depends on the pressure 

 exerted on the hair, and this in turn on the sectional area and stiffness of the 

 hair itself. One can measure the pressure exerted by pressing the hair on a 

 balance and from the sectional area of the hair deduce the pressure per sq. 

 mm. According to v. Frey the sensibility of the cornea and conjunctiva is 

 distributed in a punctiform manner, insensible areas existing between : pain 

 alone, according to v. Frey, being experienced from stimulation of the cornea 

 with the exception of its margin and the teeth, or rather the dentine and pulp. 

 (V. Frey, " Beitrage z. Physiologie d. Schmerzsinns," and "Beit. z. Sinnes- 

 physiologie d. Haut," Berich. a. d. tnath.-phys. Classe d. Konigl. Sachs. 

 Gexetl. d. Wiasen. Leipzig, Dec. 1894, and March 1895. Criticism by Nagel, 

 Pfliiffer's Archiv, Bd. 59, p. 563, 1895.) 



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

 the index-finger, as in fig. 287, 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. 



(<7.) 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 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 sensi- 

 FIG. 287. bility of different parts of the body with cold 



and warm metallic-pointed rods. 



(b.) Touch fur, wood, and metal. The metal feels coldest, 

 although all the objects are at the same temperature. 



(c.) Plunge the hand into water at 36 C. One experiences a feeling of heat. 

 Then plunge it into water at 30 C. , at first it feels cold, because heat is 

 abstracted from the hand. Plunge the other hand direct into water at 

 30 C. without previously placing it in water at 36 C., it will feel pleasantly 

 warm. 



