THE SKNSORY 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. 

 Rather 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, or the sound of a bell or whistle. 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 di- 

 rected 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. 



