422 THE NERVES AND THE SENSE ORGANS 



The nerve organ to which an afferent impulse must go, before being 

 returned as an efferent impulse, depends on the position of the asso- 

 ciating neurons for that impulse. 



Voluntary acts are directed by the cerebrum. 



Reflex acts are either natural or acquired. Acquired reflex acts are 

 habits. Natural reflex acts have their associating neurons in organs 

 below the cerebrum. Habits have their associating neurons in the 

 cerebrum, but as parts of preferred pathways, so that the turning 

 back of the impulse in the right direction requires little or no effort 

 of the mind. 



Alcohol injures the whole body by attacking the nerve cells. 



Tobacco prevents the nervous system from developing properly, 

 and is, therefore, very injurious to young people. Cigarettes are worse 

 than ordinary tobacco. 



The special senses are touch, taste, smell, hearing, and 

 sight. 



Sensations are the interpretations, made by the mind, of the afferent 

 impulses that come to the brain. There are general and special 

 sensations. 



Touch is due to nerve fibers in the touch corpuscles of the skin. It 

 is most delicate in the tongue and fingers. 



Taste is the result of stimuli received by certain nerves that end 

 in the mucous membrane of the tongue and mouth. Only substances 

 in solution can give the stimuli. 



Smell is due to impulses aroused in certain neurons by the olfactory 

 cells. 



Hearing is the effect of sound impulses set up in the audi- 

 tory nerve. This nerve ends in the cochlea, a part of the inner 

 ear. 



Sight is the sensation produced by light stimuli that are carried to 

 the brain from the retina. 



The retina is the outspread end of the optic nerve. 



The lachrymal glands secrete tears to wash the eyeballs. 



Accommodation is the power of the eye to alter the curvature of the 

 crystalline lens, so that light from both near and distant objects may be 

 brought to a focus on the retina. 



