SENSE OF TOUCH. 



SENSE OF TOUCH. 



113 



Of all the senses, Touch is the most extensively diffused through- 

 out the animal kingdom ; it is the simplest and most rudimentary of 

 all the special senses, and may be considered as an exalted form of 

 common sensation^ from which it arises by imperceptible gradations 

 till it reaches its highest development in some particular parts. It 

 is also the earliest called into operation, and the least complicated in 

 its impressions and mechanism. 



The sense of touch is most highly developed in those parts that 

 are most abundantly supplied with sensory nerves. In the lips, the 

 tip of the tongue, and the palmar aspect of the last joints of the 

 fingers, the nerves are both very numerous and superficially dis- 

 tributed, and whilst the epidermic layer is thinner, there is at the 

 same time a greater degree of isolation of the papillse of the skin 

 between lines and furrows of the epidermis. The number of these 

 lines or furrows is commensurate with the development of the sense. 

 Even in man the acuteness of the sense of touch varies much in 

 different regions of the body, as can be proved by observing the 

 varying distances at which the two points of a pair of compasses can 

 be separately recognised on different parts of the surface ; on the 

 points of the fingers they can both be recognised at a distance of \ 

 of a line, while they require to be separated 30 lines in order that 

 the two points may be recognised over the spine. 



The nerves of touch are the same as those of general sensation, 

 viz. : the posterior roots of the spinal nerves, and some fibres of the 

 fifth and eighth cerebral nerves. They are distributed to the tactile 

 papillae of the skin, small elevations enclosing loops of blood-vessels 

 and branches of the sensory nerves, (Fig. 30,) situated on the 

 exterior surface of the cutis vera. The _ 



papillce are covered by the epidermis, 

 which protects them from too violent im- 

 pressions of external bodies upon them. 



In the sense of touch the body to be ex- 

 amined must be brought into contact with 

 the sensory surface. The only exception 

 to this is in regard to the sense of tempera- 

 ture, for which there would seem to be a distinct set of nerves. 



The only idea communicated to our minds by the sense of touch, 

 is that of resistance. By the various degrees of resistance which 

 the sensory surface encounters, we obtain a knowledge of the hard- 

 ness or softness of a body. When the sensory surface, and the sub- 

 stance touched are moved upon each other, we obtain a notion of 

 extension or space. At the same time, by the impressions made 



* Papillae of the palm, the cuticle being detached. 



10* 



