37O PRACTICAL PHYSIOLOGY. [LXXVI. 



6. Sense of Contact. 



Touch your forehead with your forefinger, the finger appears 

 to feel the contact ; but on rubbing the forefinger, or any other 

 digit, rapidly over the forehead, it is the latter which is interpreted 

 as " feeling " the finger. 



1. Weber's Circles. 



Cut short lengths from glass tubing of various sizes, varying from 

 a quarter of an inch to two inches or more in diameter, and provide 

 glass vessels of similar size, each with a glass base. Press the 

 smaller circles and corresponding size of vessel on the cheek and 

 forehead and the larger ones on the thorax or abdomen. It is 

 impossible when the eyes are shut to determine whether a closed 

 or open vessel is pressed on the skin. The size of the vessel to 

 obtain this result varies with the cutaneous surface experimented 

 on. 



8. Illusions. 



(a.) Place a thin disc of cold lead the size of a florin on the forehead of a 

 person whose eyes are closed, remove the disc, and on the same spot place 

 two warm discs of equal size. The person will judge the latter to be about 

 the same weight, or lighter, than the single cold disc. 



(b.) Compare two similar wooden discs, and let the diameter of one be 

 slightly greater than that of the other. Heat the smaller one to over 50 C., 

 and it will be judged heavier than the larger cold one. 



(c.) Lay on different parts of the skin a small square piece of paper with a 

 small central hole in it. Let the person close his eyes, while another person 

 gently touches the uncovered piece of skin with cotton wool, or brings near it 

 a hot body. In each case ask the observed person to distinguish between 

 them. He will always succeed on the volar side of the hand, but occasionally 

 fail on the dorsal surface of the hand, the extensor surface of the arm, and 

 very frequently on the skin of the back. 



(d.} Estimation of the distance of two neighbouring parts depends on the 

 size of the sensory circles. If the points of a pair of compasses about I cm. 

 apart are placed on the skin in front of the ear and moved towards the lips, 

 the points feel as if they diverged. 



9. The Muscular Sense. 



(a.) With the arm and hand unsupported, the eyelids closed, and the same 

 precautions as in 3 (a.), determine the smallest difference which can be per- 

 ceived between two weights. It will be less than in cartridges filled with a 

 known weight of shot, and tested by the pressure-sense alone. The cartridges, 

 e.g., 100 grms., are numbered, and they are so made as to have a small increas- 

 ing increment of weight. They are alike in external appearance. 



(b. ) Take two equal iron or lead weights, heat one and leave the other cold. 

 The cold one will feel the heavier. 



10. Taste and Smell. Prepare a strong solution of sulphate of 

 quinine, with the aid of a little sulphuric acid to dissolve it (bitter), 



