CHAPTER XXVI. 

 THE SPECIAL SENSES. 



It is one of the functions of the nerve system to bring the individual into 

 conscious relation with the external world. This is accomplished in part 

 through the intermediation of afferent nerves, connected peripherally with 

 highly specialized terminal organs, and centrally with specialized areas in 

 the cerebral cortex. 



Excitation of the terminal organs by material changes in the environment 

 develops nerve impulses which, transmitted to the cortical areas, evoke 

 sensations. These sensations, differing in character from those Wague ill- 

 defined sensations e.g., fatigue, well-being, discomfort, etc. caused by 

 material changes occurring within the body, are termed special sensations 

 e.g., touch; pressure; pain; temperature; taste; smell; light and its varying 

 qualities, intensity, hue, and tint; sound and its varying qualities, intensity, 

 pitch, and timbre. 



The terminal organs which receive the impress of the external world are the 

 skin, tongue, nose, eye, and ear, and collectively constitute the special sense- 

 organs. The physiologic mechanisms which underlie and develop these 

 special sensations are known respectively as the tactile, gustatory, olfactory, 

 optic, and auditory. Each mechanism responds to but a single form of 

 stimulus and to no other. $?hus, the stimulus for the skin is mechanic pres- 

 sure; for the tongue, soluble organic and inorganic matter; for the nose, 

 /volatile or gaseous matter; for the eye, ether vibrations; for the ear, atmos- 

 pheric undulations. These stimuli alone are adequate to the physiologic 

 excitation of the different mechanisms. 



The factors involved in the production of the sensations include (i) a 

 special physical stimulus; (2) a specialized terminal organ; (3) an afferent 

 nerve pathway, and (4) a specialized receptive sensor cell in the cerebral 

 cortex. 



Though the resulting sensations in each instance differ widely in their 

 characteristics, it is difficult to present a satisfactory explanation for these 

 differences. If it be assumed that the nerve impulses which ascend the 

 different nerves of special sense are alike in quality, then it must be ad- 

 mitted that the character of the sensation is the expression of a specialization 

 and organization of the cortical area. If, on the other hand, specialization 

 of the cortex is denied, then there must be admitted a specialization of the 

 peripheral organ with a resulting difference in quality or rapidity of the 

 nerve impulses which would impress or excite the non-specialized cortex in 

 such a way as to call forth the characteristic sensation. It is possible, how- 

 ever, that neither supposition is wholly correct, and that the character of the 

 sensation depends on the construction and adaptation of the entire sense 

 apparatus to the character of the stimulus. 



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