iooo A. MANUAL OF PHYSIOLOGY 



hair by pressing it down upon the scale of a balance till it is slightly 

 bent, and observing the greatest weight in the other scale which it 

 will lift. Mark the number in milligrammes on the handle. In 

 this way, when a hair is placed at right angles to a point of the skin, 

 and pressure exerted on it till it begins to bend, the intensity of the 

 touch stimulus i.e., the pressure exerted on the skin is definitely 

 measured, and by using hairs of different pressure values the threshold 

 value of the stimulus for any touch area i.e., the pressure which 

 just gives the sensation of light touch can be determined (p 970). 



(a) Using the back of the hand, note how light a touch of the 

 aesthesiometer applied to the end of a hair suffices to elicit a sensa- 

 tion of touch, as compared with a part free from hairs. The hairs 

 diminish the threshold of the stimulation by acting as levers, whose 

 short arm presses against the nerve-endings surrounding the hair- 

 follicles, while the stimulating weight acts on the long arm. When 

 the skin is shaved the threshold is always raised. 



(fe) Shave an area on the back of the hand, and make out the 

 relation of the touch-spots to the hair follicles. Each hair has 

 an especially sensitive touch-spot just on the ' windward ' side of 

 the follicle (p. 970). Using aesthesiometers of different pressure 

 values, determine the threshold value for the shaved area. Outline 

 an area of a square centimetre on the skin , and determine the number 

 of touch-spots, using first a hair of the threshold value, and then 

 going over the area 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 

 examination of the skin areas. He should understand by pre- 

 liminary practice what the sensation of light touch is, the percep- 

 tion of which he is to indicate. With strong aesthesiometer hairs 

 the pricking sensation 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 tempera- 

 ture). With light contact the sensation is that of simple touch. 

 On increasing 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. 

 Repeat 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 

 aesthesiometer compasses can be recognised 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 



