CH. XIV.] REFLEX ACTION 145 



scious knowledge, so also afferent impulses may travel to the 

 central nervous system which excite no conscious feelings. The 

 afferent nerve-tracts to the cerebellum form a very good instance of 

 these. 



Then, too, the excitation of many afferent nerves will excite what 

 are called reflex actions. We are very often conscious of the sensa- 

 tions that form the cause of a reflex action, but we do not necessarily 

 have such sensations. Many reflex actions, for instance, occur during 

 sleep ; many may be executed by the spinal cord even after it has 

 been severed from the brain, and so the brain cannot be aware of 

 what is occurring. 



A reflex action is an action which is the result of an afferent 

 impulse. Thus a speck of dust falls into*the eye, and causes move- 

 ments of the eyelids to get rid of the offending object. The dust 

 excites the sensory nerve-endings in the conjunctiva, an impulse 

 travels to the centre of this nerve in the brain, and from the brain 

 a reflected impulse travels to the muscles of the eyelid. As an 

 instance of a reflex action in which secretion is concerned, take the 

 watering of the mouth which occurs when food is seen or smelt. The 

 nerves of sight or smell- convey an afferent impulse to the brain, 

 which reflects, down the secretory nerves, an impulse which excites 

 the salivary glands to activity. 



These, however, are instances of reflex action which are accom- 

 panied with conscious sensation, but like all pure reflex actions are 

 not under the control of the will. 



An instance of a reflex action not accompanied with consciousness 

 is seen in a man with his spinal cord cut across or crushed, so that 

 any communication between his brain and his legs is impossible. 

 He cannot move his legs voluntarily, and is unconscious of any 

 feelings in them. Yet when the soles of his feet are tickled he draws 

 his legs up, the centre of reflex action being in the grey matter of 

 the lower region of the spinal cord. 



For a reflex action, three things are necessary : (1) an afferent 

 nerve, (2) a nerve-centre consisting of nerve-cells to receive the 

 afferent impulse and send out the efferent impulse, and (3) an 

 efferent nerve along which the efferent impulse may travel. If the 

 reflex action is a movement, the afferent nerve is called excito-motor ; 

 if it is a secretion, the afferent nerve is called excito-secretory ; and 

 similarly, afferent nerves may also be excito-accelerator, excito-inhibitory, 

 etc. 



3. Inter-central nerve-fibres are those which connect nerve- 

 centres together ; they connect different parts of brain, and of the 

 cord to one another, and we shall find in our study of the nerve- 

 centres that they are complex in their arrangement. 



K 



