CHAPTER XX 



INTERNAL AND EXTERNAL SENSES: TASTE, SMELL, HEARING. 



AND SIGHT 



Definition of sensation, — Sensation is defined as perception 

 through the sense organs, and is the result of stimulation of these 

 organs. 



Organs necessary for sensation. — A peripheral organ for the 

 reception of a stimulus, a nerve for its conduction, and a centre 

 in the brain for the perception and interpretation are the three 

 essential parts of a sense organ. It is by means of impressions 

 received by the peripheral organs and conducted by the nerves 

 to the brain that the mind is able to control the body and to take 

 cognizance of the external world. 



Where sensations are interpreted. — Sensations are felt and 

 interpreted in the brain. Our habit of projecting sensations to 

 the part that is stimulated, tends to obscure this fact. In reality 

 we see and hear with our brains, because the eye and ear serve 

 only as end organs to receive the stimulus which must be carried 

 to the brain and interpreted before we do see or hear. 



CLASSIFICATION OF SENSATIONS 



Sensations were formerly classified into two groups, i.e. special 

 and common. The special senses were sight, hearing, touch, 

 taste, and smell. All other sensations were grouped as common. 

 A more recent classification is dependent on the part of the body 

 to which the sensation is projected, and the two groups are named : 

 (1) internal or interior senses, and (2) external or exterior senses. 

 These classifications have much in common, but differ slightly. 



Internal or interior senses are those in which the sensations are 

 projected to the interior of the body. It is by means of these 

 senses that we acquire a knowledge of the condition of our body. 

 They include hunger, thirst, pain, muscular sense, fatigue, and vari- 



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