536 MANUAL OF PHYSIOLOGY. 



localize impressions to certain points of the skin, and to project 

 into space the stimulation caused by the light reflected from distant 

 objects, so as to get a distinct and accurate idea of their position, 

 is the result of experience and habit, which teach each individual 

 that when a certain sensation is produced, it means the stimulation 

 of a certain point of the skin, and that the objects we see are not 

 in our eyes, where the impulse starts, but at some distance from 

 us. We learn this from a long series of unconscious experiments 

 carried on in our early youth by movements of the eyes with 

 cooperation of the hands. Even the sensations which arise in 

 the various centres of the sensorium, as the result of internal or 

 central excitations, are from habit attributed to external influ- 

 ences, and thus we have various hallucinations and delusions, such 

 as seeing objects or hearing sounds which only exist in the brain. 



The sensations produced in our nerve centres as the result of 

 the afferent impulses coming from our special sense organs give 

 rise to a form of knowledge called perception; each perception or 

 impulse causing an appreciable sensation, helping to make up our 

 knowledge of the outer world and of ourselves, for without this 

 power of perception we could have no notion of our own existence 

 and no ideas of our surroundings ; in fact, we should be cut off 

 from all sources of knowledge and be idiots by deprivation of all 

 intelligence from without. 



A complete special sense apparatus may then be said to be 

 made up of the following parts : 



1. A special nerve ending only capable of being excited by a 

 special adequate stimulus. 



2. An afferent nerve to conduct the impulses from the special 

 end organ to the nerve centre. 



3. Nerve cells forming a centre, which is capable by specific 

 energy of translating the nerve impulse into a sensation, and 

 which sensation is commonly referred to some local point of the 

 periphery. 



4. Associated nerve centres, capable of perceiving the sensations, 

 forming notions thereon, and drawing conclusions, from the present 

 and past perceptions, as to the intensity, position, quality, etc., 

 of the external influence. 



