74 VETERINARY PHYSIOLOGY 



hand, are either augmentor or inhibitory, but they do not act 

 at one time in the one way, at another in the other. 



B. Ingoing or Afferent Nerves. — Section of another set of 

 nerves may produce loss of sensation in some part of the 

 body. When the peripheral end of the cut nerve is stimu- 

 lated no result is obtained. When the central end is 

 stimulated, sensations, with or without some kind of action may 

 result. Such nerves obviously conduct inwards to the central 

 nervous system. Those which, when stimulated, give rise to 

 sensations may be called sensory ; those which give rise to 

 some action are called excito- reflex, because the action which 

 results is produced by reflex action (p. 82). But these two 

 are not distinct from one another, and a nerve which at one 

 time, when stimulated, causes a sensation, may at another 

 time cause a reflex action without sensation. The branch of 

 the fifth cranial nerve which passes to the conjunctiva of the 

 eye is an example of such a nerve. When the conjunctiva 

 is touched — i.e. when this nerve is stimulated — the 

 orbicularis palpebrarum is brought into action through the 

 seventh cranial nerve, and the eye is closed. The conjunctival 

 branch of the fifth cranial nerve is thus an excito-motor 

 nerve. 



When the terminations of the lingual branch of the fifth 

 nerve in the tongue are stimulated, the result is a free flow 

 of saliva, through the action of the secretory fibres of the 

 seventh nerve and of the glosso- pharyngeal. The lingual 

 nerve is thus excito -secretory. 



Stimulation of the nerves from any part — e.g. by a 

 mustard blister — causes relaxation of the blood-vessels of the 

 part, and such afferent nerves may be called excito- vaso- 

 dilator. 



C. Many nerves of the body contain both afferent and 

 efferent nerve fibres, and are called mixed nerves. 



5. The Nature of the Nerve Impulse. 



The impulse which passes along a nerve is due to 

 chansfes in the axis cvlinder, since this, without its sheaths, 

 can conduct. Further, it is dependent on the vitality of 



