The Special Senses 239 



It follows that sensation may be prevented in various 

 ways. In the sense of sight, for example, one person may 

 be blind because the terminal organ, or eye, is defective or 

 diseased. Another may have perfect eyes and yet have no 

 sight, because a tumor presses on the nerve between the 

 eye and the brain. In this case the impression fails because 

 of the break in the communication. Once more, the eye 

 may be perfect and the nerve connection unbroken, and yet 

 the person cannot see, because the center in the brain itself 

 is injured from disease or accident and cannot receive the 

 impression. 



368. The Functions of the Brain Center in the Perception 

 of an Impression. Sensation is really the result of a change 

 which occurs in a nerve center in the brain, and yet we refer 

 sensations to the various terminal organs. Thus, when 

 the skin is pinched, the sensation is referred to the skin, 

 although the perception is in the brain. We may think it 

 is the eyes that see objects ; in reality, it is only the brain 

 that takes note of them. 



This is largely the result of education and habit. Thus, 

 from the effects of a blow on the head one often sees flashes 

 of light (familiarly known as " stars ") as vividly as if torches 

 actually danced before the eyes. Impressions have reached 

 the seeing center in the brain from irritation of the optic 

 nerve, or the center itself may have been irritated, pro- 

 ducing the same effect as real lights would cause. In this 

 case, however, knowing the cause of the flashes, the injured 

 person is able to correct the erroneous conclusion. 

 ^ 369. Organs of Special Sense. The organs of special 

 sense, the means by which we are brought into relation 

 with surrounding objects, are usually classed as five in 

 number (sometimes fancifully called " the five gateways of 

 knowledge ") : the skin, the chief organ of touch ; the tongue, 



