632 THE SENSES 



deum and proctodeum. The nerves of sensation are contained in the same 

 trunks with other sensory nerves. They are found in the posterior or sen- 

 sory 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. 



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 

 cutis laid bare, the sensation of contact is replaced by one of pain. Further, 

 in all highly sensitive parts, the papillae are numerous and highly vascular, 

 and the sensory nerves are connected with special end-organs which have 

 been described on page 74 et seq. 



