CHAPTEE VIII. 



THE SENSES. 



THE senses are the endowments by which we perceive the physical 

 properties of external objects and the phenomena produced by their 

 various reactions, such as solidity, pressure, smoothness or inequality of 

 surface, temperature, light, sound, and sapid and odoriferous qualities. 

 All our information with regard to the objects of nature is obtained 

 through these channels, which are consequently the primitive source 

 of all conscious relation with the external world. Sensation alone in- 

 dicates merely the perception of some impression derived from without, 

 whatever may be its nature. The senses, on the other hand, form so 

 many subdivisions of the main function, each of which is devoted to 

 the perception of a particular class of physical properties or reactions. 

 They are divided into five different groups, namely : 1. General sensi- 

 bility. 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 that by which we appreciate the simpler physi- 

 cal properties of external objects, such as their consistency, roughness 

 or smoothness of surface, temperature, and mass. It is so called be- 

 cause it is generally diffused over the external integument, beside being 

 present in most of the mucous membranes near the surface. Notwith- 

 standing that this endowment includes the power of perceiving several 

 different kinds of impression, they are all, so far as we know, communi- 

 cated to the perceptive centres by the same nerves ; and the grade of 

 sensibility for all varies, as a general rule, in the same direction and to 

 the same degree in different parts of the body. The sensations thus 

 produced, though presenting some peculiarities by which they may be 

 distinguished from each other, are therefore naturally comprised under 

 the single head of general sensibility. 



Sensations of Touch. This is, perhaps, the least complicated form 

 of sensory impression, and is known as "tactile sensibility." It is 

 produced by the simple contact of a foreign body with the sensitive 

 surface, and gives information as to its solidity, its external configura- 

 tion, and its indifferent or irritating qualities. Although there is a 

 certain variety in these impressions, yet they evidently belong to the 

 same group, and there is no essential difference in the effect produced 

 by the contact of sharp-pointed instruments, and that caused by irri- 

 tating substances, like mustard, applied to the skin, the continuous or 



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