CHAPTER XVIII 

 THE ORGANS OF SPECIAL SENSE 



THE nerve system is the means by which the indi- 

 vidual is brought into conscious relation with the 

 external world. This consciousness is excited by 

 numerous material impressions which develop nerve 

 impulses in the end-organs of the skin, the tongue, 

 nose, eye, and ear, and are conveyed by afferent 

 nerves to the centres in the cortex of the brain where 

 they awake sensations. 



These sensations vary in character. Thus one may 

 feel happy, fatigued, hungry, thirsty, etc., as a result 

 of material changes going on within the body. These 

 are usually spoken of as common or ordinary sensa- 

 tions. The important or special sensations arise as 

 a result of the definite impressions made upon the 

 highly sensitive end-organs, as touch, pain, tempera- 

 ture, pressure, taste, smell, light and its varying 

 qualities, sound and its varying qualities. The physio- 

 logic mechanisms underlying these special sensations 

 are spoken of as tactile, touch; temperature, pain, 

 gustatory, taste; olfactory, smell; optic, sight; auditory, 

 hearing, and are known as the special senses. 



The factors necessary to the production of the 

 sensations are: (1) A special physical stimulus; (2) a 

 specialized terminal organ (end-organ) ; (3) an afferent 

 pathway which conveys the impulse to the centres 

 in the cortex of the brain; (4) a specialized receptive 

 sensor cell in the cortex of the brain. 



The special senses are five in number: sense of 

 touch, sense of smell, sense of sight, sense of hearing, 

 and sense of taste. 



