G60 HANDBOOK OF PHYSIOLOGY. 



the object itself; among which may be mentioned the number of inter- 

 vening objects, the number of steps which from past experience we 

 know we must take before we could touch it, and many others. 



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 a certain degrees, the seat of this sense ; but touch should not be 

 considered as a mere modification or exaltation of common sensation or 

 sensibility. For although the nerves on which the sense of touch de- 

 pends, are 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, yet it 

 seems probable that the nerve-fibres which subserve the special sense of 

 touch are provided with special end organs. 



All parts of the body supplied with sensory nerves are thus, in some 

 degree, organs of touch, yet the sense is exercised in perfection only in 

 those parts the sensibility of which is extremely delicate, e.g., the skin, 

 the tongue, and the lips, which are provided with abundant 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 eye- 

 lashes. 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 sensa- 

 tion. 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 fibres are intensely affected in a small extent; the sensation 

 of pressure indicates a slighter affection of 'the parts in the greater ex- 

 tent, 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 proper, tactile sensibility or pres- 

 sure, (b) temperature. These when carried beyond a certain degree are 

 merged in the sensation of (c) pain. 



Touch proper. In almost all parts of the body which have deli- 

 cate tactile sensibility the epidermis, immediately over the papillae, is 

 moderately thin. When its thickness is much increased, as over the 

 heel, the sense of touch is very much dulled. On the other hand, when 

 it is altogether removed, and the cutis laid bare, the sensation of con- 



