PRACTICAL EXERCISES 1115 



again with a hair of a decidedly higher pressure value. The threshold 

 value for many parts of the hairy skin is obtained with a hair which 

 bends at 70 milligrammes. Repeat the determinations for other skin 

 areas, such as the back of the upper arm, the palm of the hand, the 

 anterior surface of the leg, the chest, the back, and the cheek, forehead, 

 and lips. 



It is well that the subject should be blindfolded during the ex- 

 amination of the skin areas. He should understand by preliminary 

 practice what the sensation of light touch is, the perception of which 

 he is to indicate. With strong sesthesiometer hairs the pricking sen- 

 sation due to stimulation of pain-spots must be discriminated from 

 touch sensation. When the two sensations are elicited together, the 

 touch sensation is momentary, and the subject must be alert to detect 

 it immediately on stimulation. The pain sensation develops more 

 slowly, but lasts longer and becomes much more conspicuous than the 

 touch sensation, which accordingly is apt to be submerged by it in 

 consciousness. 



(2) Touch the skin with a blunt point (at or about skin temperature). 

 With light contact the sensation is that of simple touch. On in- 

 creasing the pressure, the quite distinct sensation of deep pressure is 

 perceived. 



(3) Touch a portion of skin with a camel's-hair brush of ordinary 

 size, pressing on it till the hairs of the brush begin to bend. The first 

 sensation of simple contact gives place to a sensation of pressure. Re- 

 peat with a camel's-hair brush of the finest hairs half a centimetre in 

 length, cut away till its cross section is only half a millimetre in diameter 

 at the base. Probably a pure sensation of touch, without any pressure 

 element, will be obtained when the brush is applied so as just to bend 

 the hairs. 



(4) Find the least distance apart at which the points of the aesthesi- 

 ometer compasses can be recognized as two when applied to the back of 

 the hand, the forearm, upper arm, forehead, finger-tips, or tip of the 

 tongue. Both points of the compasses must be placed on the skin 

 at the same time, and the same pressure applied to both. The subject 

 must not see the points. 



(5) Time Discrimination of Touch. Touch the prong of a vibrating 

 tuning-fork lightly with the. tip of the finger. The taps of the prong 

 on the skin do not blend into a continuous sensation even when the 

 fork vibrates several hundred times per second. 



30. Temperature Sensations. For the investigation of these, pieces 

 of thick copper wire, filed at one end to a blunt point, and fixed by 

 the other in a small wooden handle, may be used. They can be heated 

 in a sand-bath or in a beaker of water to the desired temperature, or 

 cooled in cold water or in ice. Or a metal tube drawn out at one end, 

 through which water at the required temperature can be passed before 

 use, may be employed. Another device is a metal cylinder ending in 

 a point, and filled with water at the given temperature. 



(i) On the dorsal side of the hand outline an area of skin with a 

 pen or a coloured pencil. Divide this into areas of 4 square milli- 

 metres. Go over the area with a wire or cylinder at a temperature of 

 about 40 C., and determine the extent and position of the spots which 

 on contact yield a sensation of warmth, marking them on the skin by 

 ink-dots, or mapping them on ruled paper. Then repeat the ex- 

 ploration with points at a temperature of about 15 C., and map the 

 spots which yield a sensation of coolness. Now note whether a warm 

 spot touched with a point at 15 C., or a cold spot touched with a point 

 at 40 C., yields any temperature sensation. 



