TACTILE SENSE. 



725 



2. The minimum limit varies for the separate sensations, or, 

 rather, the single specific agents. Thus, the minimum for excitation 

 of touch is a pressure of 0.002 milligrams; for temperature, V 8 C.; 

 for sensation of movement, a shortening to the extent of 0.044 milli- 

 meters of the internal rectus of the eye ; for hearing, the noise made 

 by a ball of pith one milligram in weight falling one millimeter in 

 height upon a glass plate heard at a distance of ninety-one milli- 

 meters from the ear; for sight, an intensity of light about three 

 hundred times feebler than that of the full moon. 



3. The intensity of the sensation is proportional to the intensity 

 of the stimulus and the degree of irritability of the nerve at the 

 moment of excitation. As the strength of the stimulus increases, so 

 do the sensations. But the sensations increase equally when the 

 strength of the stimulus increases in relative proportions. Thus, 

 small noises will be distinguished in the silence, not in the midst of 

 loud noises; a slight difference will be noticed between small weights, 

 not between heavy ones. A burning candle in the daytime makes 

 little impression. 



4. Sensations do not increase in the same proportion as the 

 stimulus. If the stimulus increases in geometrical progression, then 

 the sensation increases in simple arithmetical progression. Eather, 

 it increases as the logarithm of the strength of the stimulus. (This is 

 Fechner's psycho-physical law.) 



5. For the single, specific sense apparatuses, whenever a stimulus 

 takes place, whether at the peripheral terminations of a nerve or in 

 its course, or at its central point, the individual always localizes with 

 Ids perception the stimulus at the place where the normal stimulus 

 operates. That is, for sight and hearing he refers it to space ; for the 

 nerves of taste, smell, or touch, he refers it to the peripheral regions 

 of his body, even if these be lacking. Thus, in an amputated leg, pain 

 in the stump is referred to the toes. This is the law of eccentric 

 projection of sensation. 



Touch. 



The organ of touch is represented by the skin and -mucous mem- 

 branes in proximity to the natural orifices of the body. 



The skin, or common integument, is composed of the following 

 layers: (1) the epidermis; (2) the corium, or cutis vera, with its 

 papillae; and (3) the subcutaneous tissue with the adipose tissue. 



1. The Epidermis belongs to the tissues which are composed of 

 simple cers united to each other by cement-substance. It in itself 

 consists of several layers. 



