SPECIAL SENSES. 535 



nals are known as the special sense organs, the physiology of which 

 is one of the most difficult and most interesting branches of study 

 in Biological Science. 



The nerves which carry the impulses from the various organs 

 of special sense do not differ from other nervous cords, so far as 

 their structure and capabilities are concerned. But besides their 

 special end organs they are connected with nerve cells in the 

 brain, the sole duty of which is to receive impulses from one of 

 the special sense organs and convert the same into a special sen- 

 sation. No matter by what means a nerve trunk from a special 

 sense organ be stimulated, the impulse excites in the sensorium 

 the sensation usually arising from stimulation of the special organ 

 to which it belongs. Thus electric stimulation of nerves in the 

 tongue causes a certain taste, mechanical or other stimulation of 

 the optic nerve trunk gives rise to the sensation of flashes of light, 

 and a persistent odor may be caused by the presence of a bony 

 growth, pressing upon the olfactory nerve. 



The capability of the nerve centres connected with the nerves 

 of special sense to give rise invariably to a special sensation, 

 is called their specific energy. And the special influence, light, 

 sound, etc., which alone suffices to excite the special peripheral 

 terminal, and which the given terminal alone can convert into a 

 nerve stimulus, may be called its specific or adequate stimulus. 



Although we habitually refer the sensation to the surface where 

 the stimulus is applied, as if we really felt with our skin, and 

 recognized sound sensations with our ears, etc., the sensation only 

 occurs in the centres in the brain. This is obvious from what 

 has been already said of the nerve fibres of the special sense 

 organs, namely, that if a stimulus be applied to the nerve trunk, 

 the same sensation is produced as if the specific stimulation had 

 operated on the special nerve terminal frmn which these fibres 

 habitually carried impressions. This peripheral localization of 

 sensations is really accomplished in the mind, just as by a mental 

 act of a different character, the impressions communicated by the 

 eye are projected into the space about us in our thoughts, instead 

 of being referred to the retina, or thought of as being produced 

 in the eye itself. This power of the centres of the sensorium to 



