298 THE NERVOUS SYSTEM 



have been gradually made out. While our knowledge of 

 the functions of the nervous system has thus been greatly 

 extended, especially during recent years, it is probable 

 that a much larger field still remains only partly explored, 

 and there is reason to expect that a much more accurate 

 and definite acquaintance with this most important portion 

 of the human organism will soon become possible. 



432. Functions of the Nerves. The nerves are the tele- 

 graph wires of the system. They transmit impulses 

 from all parts of the body to the central nervous system 

 and from the central system to all parts of the body. 

 Individual nerve fibers transmit impulses in only one 

 direction. 



433. Afferent and Efferent Nerves. Nerves or nerve 

 fibers which carry nervous impulses to the centers are 

 afferent; those which carry impulses from the centers are 

 efferent. The first are generally called sensory ; but im- 

 pulses which do not result in sensation, that is, which do 

 not affect consciousness, may be conveyed by afferent 

 nerves. The spinal cord may be cut or injured so that 

 its lower portion and the nerves issuing from it have no 

 connection with the brain. Still if the foot is pricked or 

 tickled, the afferent nerves carry the impulse to the cord, 

 efferent nerves bring back a motor impulse, and the foot 

 is drawn back, all without conscious sensation. 



434. Classification of Nerves. Afferent nerves have been 

 classed in two groups. 



1. Nerves of special sense, viz. of sight, hearing, touch, 

 taste, and smell. 



2. Nerves of general sensibility. These convey to the 

 brain those vague, indescribable feelings of comfort and 

 general well-being, or of discomfort, languor, and weari- 

 ness, which come from the interior of the body. 



