CHAPTER VIII. 

 THE SENSES. 



THE senses are the endowments by which we gain perception of 

 external objects and phenomena. They are consequently the primi- 

 tive source of all information, and the channels of all conscious relation 

 with the external world. The term sensation indicates the perception 

 of any impression from without, of whatever nature. The senses, on 

 the other hand, are subdivisions of the main function, each devoted to 

 a particular class of phenomena. They are five in number, namely : 

 1. General sensibility. 2. The sense of taste. 3. The sense of smell. 

 4. The sense of sight. 5. The sense of hearing. 



General Sensibility. 



General sensibility is the faculty by which we appreciate the simpler 

 physical properties of external objects, such as their consistency, sur- 

 face, temperature, and mass. It exists throughout the general integu- 

 ment, and in the mucous membranes near the exterior. Notwithstand- 

 ing that it includes several different impressions, they are all, so far as 

 we know, communicated by the same nerves ; and the grade of sensi- 

 bility for all varies in the same direction and to the same degree in 

 different parts of the body. The sensations thus produced, though 

 presenting certain differences from each other, are therefore associated 

 under the head of general sensibility. 



Sense of Touch. This is, perhaps, the simplest form of sensory im- 

 pression, and is known as " tactile sensibility." It is produced by the 

 contact of foreign bodies with the sensitive surface, and gives informa- 

 tion as to their solidity, configuration, and indifferent or irritating quali- 

 ties. There is a certain variety in these impressions, but they evidently 

 belong to the same group. There is no essential difference in the 

 effect of sharp-pointed instruments or irritating substances applied to 

 the skin, the passage of the galvanic current, pungent liquids in con- 

 tact with the tongue, or pungent vapors in the nasal passages. They 

 are all impressions of tactile sensibility, and depend on a similar irri- 

 tation of the nervous extremities. 



The grade of tactile sensibility varies in different regions. The method 

 adopted for appreciating this variation consists in applying to the skin 

 or mucous membrane the points of a pair of compasses, tipped with 

 small pieces of cork. If the two points be a very short distance apart, 

 they cannot be accurately distinguished from each other, and the two 

 sensations are blended into one. The minimum distance at which the 



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