PRACTICAL EXERCISES 1071 



(2) Prepare a series of solutions of sulphuric acid of gradually in- 

 creasing strength, beginning with a - - solution (a two-thousandth 



gramme-molecular solution) (p. 420). Put into the mouth, after 

 previous rinsing with distilled water, 4 or 5 c.c. of one of the solutions 

 of the acid, beginning with the weakest, and determine at what con- 

 centration of the H ions the acid taste first appears, rinsing out the 

 mouth after each observation. Repeat the experiment with solutions 

 of hydrochloric acid, and determine whether the threshold value is the 

 same. 



A similar comparison of the necessary concentration of the OH 

 ions can be made with solutions of sodium hydroxide and potassium 

 hydroxide. 



(3) Connect two short pieces of platinum wire with the coppep wire 

 from the poles of a Daniell or dry cell. Apply one platinum wire to 

 the inner surface of the lip and the other to the tip of the tongue. 

 Reverse the poles. Note the difference in the sensation according to 

 whether the anode or the kathode is on the tongue. 



28. Smell. (i) Pass a current through the olfactory mucous mem- 

 brane by connecting one electrode with the forehead and the other 

 by means of a small piece of sponge or cotton-wool soaked in physio- 

 logical salt solution with one nostril. An odour like that of phosphorus 

 will be perceived. 



(2) To distinguish between Taste and Smell. Use a solution of clove- 

 oil in water which can just be distinguished from water when it is placed 

 on the tongue by means of a camel's-hair brush. Close the nostrils, 

 and determine whether the clove-oil can now be detected. 



29. Touch and Pressure. (i) Prepare a number of hair aesthesio- 

 meters by fastening hairs of different thicknesses to small wooden 

 handles about 3 inches long by means of sealing-wax. Hairs as straight 

 as possible should be chosen, or straight portions of hairs. The hair is 

 to be fastened on one end of the piece of wood at right angles to the 

 long axis of the handle, so that about an inch of the hair projects to 

 one side. Determine the pressure value of each 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. 1041). 



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

 ometer applied to the end of a hair suffices to elicit a sensation 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. 



(b) 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. 1040). 

 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 



