CHAPTER VIII 



THE SENSE ORGANS 



PAGES 



PART 1 .Introduction .... . 478 



PART 2. Vision . . 486 



PART 3. Hearing ... 595 



PART 4. Speech and voice . . . . . . . . . . .618 



PART 5. Cutaneous sensations . . . . 627 



PART 6. Taste and smell .... .640 



PART 7. Movement and position sense . ... 647 



PART 8. Labyrinth sense 652 



PAET I 

 INTRODUCTION 



THE ability to originate sensations is common to almost all parts of the 

 body. Thus the muscles, joints and viscera send nerve impulses to the 

 brain, which record their well-being and activities. We can classify 

 the sense organs into two main groups, those which belong to the common 

 sensibility of the body, and those which form the special senses. The char- 

 acteristics of the. former are (1) that they rarely send impulses, other than 

 those of pain, which pass the threshold of consciousness ; (2) that they play 

 a very large part in the initiation of the numerous reflex actions ; (3) that 

 when their impulses do reach consciousness they lack definition. The 

 characteristics of the special senses on the other hand are (1) that they very 

 frequently send impulses which reach consciousness ; (2) that the con- 

 nection between the stimulus and the resulting response on the part of the 

 individual (as expressed by movements of limbs, etc.) is usually not of a reflex 

 nature ; (3) that the information supplied to consciousness is very definite 

 both as to quality and intensity. 



The indefiniteness of the connection between stimulus and response causes 

 us to depend almost entirely on introspection in our study of the sense- 

 organs. But introspection as a method of research has grave disadvantages, 

 for we cannot measure our sensations in terms of physical units. We cannot 

 say how red a rose is or how nice it smells. Neither can we compare the 

 intensity of a beam of light when we feel its warmth on our skin with the 

 strength of the stimulus which we receive when it enters our eye. Still 

 less can we judge accurately of the sensations evoked in other individuals 

 or animals when we apply stimuli to them. , Because of this difficulty 



478 



