THE SENSORY CENTERS OF THE BRAIN 883 



such can be recognized. A case is also described in which vision persisted 

 in one half of the visual field on recovery from an occipital injury, and 

 yet things which were seen quite well could not be oriented in space, 

 and thickness and depth found no place in the visual perception. 



The sensory areas of the cortex and thalamus are the end points to 

 which we trace the afferent impulses which give rise to sensation. We 

 are not justified, however, in concluding from this that they are the 

 regions in which the phenomena of sensation and consciousness occur. 

 Bather should they be 'thought of as important junction points on the 

 afferent side of the complex network of neurons which links up the 

 various centers of the cerebrum and in which are carried out our mental 

 processes, which give rise to consciousness. In a similar way the motor 

 centers which we are to consider in the next chapter are the junction 

 points from which start out the efferent impulses for voluntary move- 

 ment. 



Sensory Hallucinations. It seems probable that under pathological 

 conditions disturbances may be set up locally in the sensory centers 

 which resemble closely those naturally occurring as the result of peri- 

 pheral stimuli. Thus in Jacksonian epilepsy the irritation arising from 

 a splinter of bone pressing upon one of the sensory centers may give 

 rise to vague sensations or aura, such as flashes of light, loud noises, or 

 tingling in the skin. If such disturbances resemble closely enough those 

 occurring naturally, they may give rise to those conscious phenomena known 

 as hallucinations and the sensory disturbance manifests itself as a definite 

 vision, the sound of a bell or whistle, or a sensation of taste or smell. 

 Use has been made of the fact that hallucinations may be set up by 

 cortical stimulation, in tracing out the sensory areas in animals. If an 

 irritant such as strychnine be applied locally to the cortex, the sensation 

 is referred by the animal to the corresponding portion of the body and 

 an effort made which is directed toward removing the irritant from this 

 region. Thus irritation of certain regions in the cortex will cause the 

 animal to shake one paw and attempt to brush away from it the supposed 

 source of the sensation. 



