Chap. XIX] THE NERVOUS SYSTEM 399 



the individual thus being totally oblivious to the reflex act. 

 Even if the sensory impulse goes to the brain, the consciousness 

 of the sensation by the individual is always later in point of time 

 than the reflex act. For example : If, without a person's 

 knowledge that the experiment is to be tried, one's finger be 

 pricked with a pin, the finger is instantly pulled back and the 

 act is done before the individual is conscious of the pain. In 

 this experiment the sensory impulse of the pin prick passed to 

 the spinal cord, set up the motor action necessary to withdraw 

 the finger, and then passed on to the brain. Again, many 

 sensory impulses produce their reflex without the brain bother- 

 ing about the matter at all. An example of this is the act of 

 walking. Walking is an exceedingly difficult accomplishment 

 to learn, acquired in childhood only after laborious eft'ort, not 

 because the muscles are weak, but because the human individual, 

 when erect, is in an exceedingly unstable state of equilibrium, 

 and constant contraction and relaxation of groups of muscles 

 is necessary to maintain the balance. Here the sensory impulses 

 of being out of balance arouse motor impulses in first one set of 

 muscles, then another, to restore the balance. At first this is only 

 accomplished with mental appreciation of the performance ; later 

 on one learns the trick, and the act of walking or standing upright 

 is performed without a moment's thought or even consciousness 

 of the difficult task we are doing purely by reflex activity. 



The kind of stimulus which will call forth the nerve-impulse 

 depends on the peripheral termination of the afferent nerve, and 

 the kind of response which an appropriate stimulus will call 

 forth depends on the mode of termination of the efferent nerve. 

 Thus, light falling on the retinal coat of the eye (the peripheral 

 termination of the sensory nerve) generates an impulse which 

 passes to the centre by the optic nerve, and returns again by the 

 motor oculi nerve to the periphery ; viz. the sphincter of the iris 

 (the termination of the motor nerve), which by its contraction 

 narrows the pupil. Hence arises the well-known phenomenon of 

 the contraction of the pupil when light falls upon the eye. 



Also stimulation of taste fibres in the mouth causes a reflex 

 secretion of the salivary glands. Innumerable examples of this 

 kind might be given. Indeed, since physical life has been well- 

 defined as the continual response to external stimuli, reflex 



