DETERMINATION OF THE FUNCTION OF A NERVE. 463 



Sensory Nerves. When these nerves are stimulated an impulse 

 is carried to a nerve-center ; if this center is the brain, the sen- 

 sation may be a conscious one, and may or may not be painful. 



Nerves of Special Sense. The impulses carried by these nerves 

 do not give rise to pain, but with each nerve is connected a special 

 sensation : With the olfactory, the sense of smell ; with the optic, 

 the sense of light ; and with the auditory, the sense of hearing. 



Thermic Nerves. It is believed by some writers that there are 

 special nerves which convey the sense of temperature only ; but 

 this is still an unsettled question. 



Exeitoreflex Nerves. In these nerves there is an impulse car- 

 ried to a nerve-center without producing a conscious sensation : 

 this center is excited, and from it or from another center with 

 .which it is in communication there goes out an impulse that, if it 

 is a gland to which it is distributed, produces secretion : such a 

 nerve would be an excitosecretory nerve. Or if it is distributed to 

 a muscle, it produces motion, and would in that case be considered 

 an excitomotor nerve. 



Afferent Inhibitory Nerves. The afferent inhibitory nerves are 

 also called centro-inhibitory, to distinguish them from the efferent 

 inhibitory nerves. The centro-inhibitory nerves carry impulses to 

 nerve-centers, which are so affected as to prevent them from send- 

 ing out impulses. A familiar instance is that of pinching the lip 

 to prevent sneezing. It is, however, doubtful whether there exists 

 a separate class of nerves performing this function, rather than 

 ordinary sensory fibers which act in this peculiar manner for the 

 moment. 



Intercentral Nerves. The nerve-centers are intimately con- 

 nected with one another by nerves which are neither afferent nor 

 efferent, and which are called intercentral. As has been said, even 

 the simplest movements of the body bring into action several, and 

 sometimes many muscles ; of course, this action is more obvious 

 in complex movements. To accomplish this, various nerve-centers 

 must be at work ; and that they may act harmoniously and pro- 

 duce coordinated movements it is essential that they should be in 

 intimate relationship. Study for a moment the intricate mechanism 

 brought into play in the ordinary act of picking up a pin from 

 the floor, and it will be readily understood how essential it is that 

 the nerve-centers responsible for these movements should act in 

 the most perfect harmony, sending to each muscle just the right 

 amount of nerve-force and at exactly the right moment ; other- 

 wise the act could not be accomplished in the perfect manner 

 that it is. 



Determination of the Function of a Nerve. The func- 

 tion of a nerve may be determined by (1) dividing it, and observ- 

 ing what function has been lost ; or (2) stimulating it, and observing 

 the effect of the stimulation. Thus when a motor nerve is divided, 



