86 CONSCIOUS NERVOUS OPERATIONS 



are of this sort. It is supposed that these are associated 

 with the ramifying, interlacing plexuses of nerve fibrils in 

 many parts of the body, and are not due to the excitation 

 of specially constructed nerve endings. 



Other sensations are more definite. We judge them to 

 be caused by some influence acting upon particular parts 

 of the body. What have long been known as the five 

 senses are of this sort. But we now recognize, in addi- 

 tion to touch, taste, smell, sight, and hearing, other sensa- 

 tions apparently distinct from them, for which, in some 

 cases at least, special nerve endings are provided, and spe- 

 cial brain centers. These are sensations of temperature, 

 of pain, of hunger, and of thirst, and the muscular sensa- 

 tions previously mentioned ( 95). 



112. The Sense of Touch. Touch has been defined as a 

 sense of pressure referred to the surface of the body. It is 

 that sort of impression upon the nervous system which 

 gives information respecting certain properties of bodies 

 in contact with the skin or mucous membrane. Through 

 it we learn whether an object is hard or soft, rough or 

 smooth, and other particulars, some of which are also given 

 by the sense of sight. 



The skin is supplied with a variety of special adapta- 

 tions which constitute it the organ of touch. 



113. The Nervous Apparatus for Touch. The thirty-one 

 pairs of spinal nerves contain the fibers for feeling for 

 the larger part of the body, as well as most of the motor 

 nerve fibers supplying the muscles. It will be remem- 

 bered that each spinal nerve arises by two roots from the 

 spinal cord, one root containing afferent or sensory fibers, 

 the other efferent or motor fibers. By the union of the 

 two roots the nerve is formed, which thence contains both 

 sorts of nerve fibers, as do its large branches. 



