THE TISSUES 83 



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-rejlex, because the action which results is produced by 

 what is called reflex action. But these are not distinct from 

 one another, and a nerve which at one time when stimulated 

 will cause a sensation, may at another time cause a reflex 

 action without sensation. 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 branch of the fifth 

 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-pharyugeal. 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-inhibitory. 



Many nerves of the body contain both afferent and efferent 

 nerve fibres, and are called mixed nerves. 



The passage of excitation from one neuron to others in such 

 actions occupies a very appreciable time. 



In the case of reflex closure of the eye, about '06 second 

 elapses between the touching of the eye and the resulting 

 " wink." 



Knowing the rate at which nerve changes pass along nerves, 

 and knowing the length of the ingoing and of the outgoing 

 leurons, the time taken in the passage of the change along 

 these is readily calculated. In a reflex wink it is about 

 '01 second. 



Hence only one-sixth of the total " latent time " of the reflex 

 action is occupied in the passage of the change along the 

 neurons, and -05 second, or five-sixths of the whole is taken up 

 in the passage of the change from one neuron to another. 

 Obviously the synapsis between the dendrites of the neurons 



