366 SPECIAL PHYSIOLOGY. 



objects, is obtained in two ways. Either we judge by the relative 

 distance and position of two or more, or even of many, points of the 

 sensory surface of the part of the body employed in the act of feeling, 

 with the motion or situation of which, we are accurately informed by 

 the cutaneous sense of locality; or we touch in succession several 

 points of the objects to be examined, with the same sensory surface of 

 the body, and then by the extent and direction of the movements 

 necessarily performed by us in these acts, judge of the size and form 

 of the object in question. In the former case, our perceptions become 

 more accurate when the acts of contact with the foreign body are suc- 

 cessive, than when such contacts take place simultaneously ; moreover, 

 the acts of contact, when successive, must neither be too quick nor too 

 slow, or we lose the power of judgment from them. In these applied 

 uses of the sense of touch and the muscular sense, for the formation 

 of compound ideas and notions, we are, in practice, greatly and habit- 

 ually assisted by the sense of sight. 



Long-continued impressions On the nerves of touch leave after, or 

 secondary impressions, which sometimes persist for a long period; as, 

 when a person in the habit of wearing a ring, believes that he still 

 feels it, after he has left it off. These after impressions are dependent 

 on some altered condition of the skin. The general after effects of 

 continued and successive tactile impressions are remarkable. Thus, 

 if the hand is brought into contact with a rapidly revolving disc, the 

 edge of which is provided with uniform fine teeth, alternate and dis- 

 tinct tactile sensations of contact and non-contact are produced. With 

 a certain velocity, these remain distinct, but at very high velocities, 

 the two sensations become less so, and resemble in character those 

 known as rough or woolly. Still more rapid revolutions of the disc at 

 length produce uniform sensations, which are comparable with that of 

 smoothness or even with that of complete polish. (Valentin.) 



Continued uniform pressure upon any portion of the skin ceases, 

 after a time, to produce any impression; but when the pressure is no 

 longer uniform, i. e., if it be lessened or augmented, its presence is 

 immediately noticed. If the pressure, after having been uniformly 

 kept up for any length of time, be removed, an after sensation, as 

 usual, remains. 



The mind, as already mentioned, generally refers the sensations of 

 touch to the part of the skin stimulated, but, under certain circum- 

 stances, they are referred altogether to the exterior. Thus, when an 

 object is brought into contact with parts destitute of nerves, like the 

 hairs or nails, the effect is communicated through them to the sensory 

 .portion of the skin from which they grow, and gives rise to sensations 

 which are distinctly referred to the insensible hair or nail. In the 

 same manner, the sensations of touch are sometimes referred to the 

 extremity of a foreign body in contact with the skin ; thus, if a stick 

 be held by one end, and its other end be brought into contact with 

 any object, we perceive a twofold impression, one where the stick is 

 held, and another which is referred to the end of the stick touching 

 .the foreign body. 



The sense of touch can be excited, as it is said, subjectively, or by 



