THE TISSUES 85 



activity. The former class may be called augmentor nerves, 

 the latter inhibitory nerves. 



The augmentor nerves may further be divided into groups 

 according to the structures upon which they act. Those act- 

 ing on muscle may be called motor nerves ; those acting to 

 cause secretion from a gland, secretory nerves ; those acting 

 to constrict blood vessels, vaso-constrictor nerves. 



The inhibitory nerves may be similarly subdivided into 

 musculo-inhibitory, secreto-inhibitory , and vaso-inhibitory 

 nerves. 



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 stimu- 

 lated, sensations or some kind of action results. Such nerves 

 obviously conduct 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 

 what is called reflex action. As an example of such a nerve 

 we may take the branches of the fifth cranial nerve which 

 pass to the conjunctiva of the eye. 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 nerve in the tongue 

 are stimulated the result is a free flow of saliva, through the 

 action of the seventh nerve and the secretory branches of the 

 glosso-pharyngeal. The lingual nerve is thus excito-secretory. 

 Further stimulation of the nerves from any part e.g. by a 

 mustard blister causes relaxation of the vessels, and such 

 afferent nerves may be called excito-vaso-inhibitory. 



A consideration of these various examples shows that there 

 is no hard and fast distinction between sensory and excito- 

 reflex nerves. Many produce both sensation and reflex action. 

 Some may at one time produce sensation alone, at another 

 reflex action alone. 



Many nerves of the body contain both afferent and efferent 

 nerve fibres, and are called mixed nerves. 



