682 THE SENSES 



like the organ of sight, for example, but is found in all parts of the skin and its 

 inversions-, the stomodeum and proctodeum. The nerves of touch sensa- 

 tion are contained in the same trunks with other sensory nerves. They are 

 found in the posterior or sensory roots of the spinal nerves and in the sensory 

 divisions of the cranial nerves, especially the fifth, seventh, ninth, and tenth. 

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

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

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

 the hands, the tongue, and the lips, which are provided with abundant touch 

 papillae. A peculiar and very acute sense of touch is exercised through the 

 medium of the nails and teeth, and, to a less extent, the hair may be consid- 

 ered an organ of touch, as in the case of the eyelashes. It has been computed 

 that the human body possesses over 500,000 touch spots. 



FIG. 420. Touch Corpuscle. 



The sense of touch renders us conscious of the presence of a contact 

 stimulus, from the slightest to the most intense degree of its action. The 

 modifications of this sense often depend on the extent of the parts affected. 

 The sensation of pricking, for example, is produced when the sensitive fibers 

 are intensely affected in a small extent; the sensation of pressure indicates 

 a slighter affection of the parts over a greater extent and depth. It is by the 

 depth to which the parts are affected that the feeling of pressure is distin- 

 guished from that of mere contact. 



In almost all parts of the body which have delicate tactile sensibility the 

 epidermis, immediately over the dermal 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 



