CHAPTER XXIV. 

 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 through the intermediation of afferent nerves, con- 

 nected 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 vague 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; pres- 

 sure ; pain ; temperature; taste; smell; light and its varying quali- 

 ties, 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 con- 

 stitute 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. Thus, the stimulus for the skin is mechanic pressure; 

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

 volatile or gaseous matter; for the eye, ether vibrations; for the 

 ear, atmospheric 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; and (4) a specialized receptive sensor cell. 



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 admitted that the character of the sensation is the ex- 

 pression 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 



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