172 PRINCIPLES OF PHYSIOLOGY 



calibre as occurs when a vaso-motor nerve is 

 acted on ; (4) pain might be felt when the 

 nerve is sensory and carries impulses to the 

 brain ; (5) if it were a nerve of special 

 sense, such as the optic or the auditory 

 nerve, there would be a sensation of light or 

 colour, or sound ; (6) in an electric fish, the 

 result might be an electric shock from the 

 electric organ These phenomena are often 

 complicated. Sometimes we have a nerve 

 that has only one function, that of causing, 

 say, motion or secretion, but usually a large 

 nerve consists of fibres having different 

 functions. For example, a nerve may contain 

 both motor and sensory fibres, and might serve 

 in part to excite movement, and in part to 

 convey impressions of touch or temperature or 

 pain. When a fibre is stimulated no physical 

 change can be seen with even the highest 

 powers of the microscope. Nerves may also 

 be conveniently classified for physiological 

 purposes into (a) centrifugal, those conveying 

 impulses from nerve centres outwards, and 

 centripetal, or those carrying impulses from 

 the outer parts of the body to nerve centres. 



