550 HANDBOOK OF PHYSIOLOGY. 



THE SPECIAL SENSES. 



I. Touch. 



Seat. The sense of touch is not confined to particular parts of the 

 body of small extent, like the other senses; on the contrary, all parts 

 capable of perceiving the presence of a stimulus by ordinary sensation 

 are, in certain degrees, the seat of this sense; for touch is simply a modi- 

 fication or exaltation of common sensation or sensibility. The nerves 

 on which the sense of touch depends are, therefore, the same as those 

 which confer ordinary sensation on the different parts of the body, viz., 

 those derived from the posterior roots of the nerves of the spinal cord, 

 and the sensory cerebral nerves. 



But, although all parts of the body supplied with sensory nerves are 

 thus, in some degree, organs of touch, yet the sense is exercised in per- 

 fection only in those parts the sensibility of which is extremely delicate, 

 e. g., the skin, the tongue, and the lips, which are provided with abun- 

 dant papillae. A peculiar and, of its own kind in each case, a very 

 acute sense of touch is exercised through the medium of the nails and 

 teeth. To a less extent the hair may be reckoned an organ of touch; as 

 in the case of the eyelashes. The sense of touch renders us conscious of 

 the presence of a stimulus, from the slightest to the most intense degree 

 of its action, by that indescribable something which we call feeling, or 

 common sensation. The modifications of this sense often depend on the 

 extent of the parts affected. The sensation of pricking, for example, 

 informs us that the sensitive particles are intensely affected in a small 

 extent; the sensation of pressure indicates a slighter affection of the 

 parts in the greater extent, and to a greater depth. It is by the depth 

 to which the parts are affected that the feeling of pressure is distinguished 

 from that of mere contact. 



Varieties. (a) The sense of Touch, strictly so-called (tactile sensi- 

 bility), (b) the sense of Pressure, (c) the sense of Temperature. 

 These when carried beyond a certain degree are merged in (d) the sen- 

 sation of Pain. 



Various peculiar sensations, such as tickling, must be classed with 

 pain under the head of common sensations, since they give us no infor- 

 mation as to external objects. Such sensations, whether pleasurable or 

 painful, are in all cases referred by the mind to the part affected, and 

 not to the cause which stimulates the sensory nerves of the part. The 

 sensation of tickling may be produced in many parts of the body, but 

 with especial intensity in the soles of the feet. Among other sensations 

 belonging to this class, and confined to particular parts of the body, may 

 be mentioned those of the genital organs and nipples. 



(a) Touch proper. In almost all parts of the body which have 

 delicate tactile sensibility the epidermis, immediately over the papillae, 



