CHAP. XIX.] ORGANS OF SPECIAL SENSE. 219 



vidual will be unable to see with that eye. In thinking of the 

 sensory organs, then, we must remember that they are structures 

 specially modified and adapted to receive certain impressions, 

 which impressions, when conveyed to the brain, give rise to definite 

 sensations. 



The sense of touch. The sense of touch, as also the sense of 

 heat and cold, is possessed more or less acutely by all parts of 

 the skin, and also by the mucous membrane, or internal skin, 

 lining the nose and mouth. We noted in the lesson on the 

 skin, that the sense of touch is most delicate in those parts 

 where the papillae of the true skin are most abundant and con- 

 tain the special nerve-endings called tactile corpuscles, as in the 

 tips of the fingers and toes (vide page 191). The feelings of 

 warmth and cold are also caused by exciting sensory nerves dis- 

 tributed to the skin, though they are probably distinct from 

 those which give rise to the sense of touch. The sense of 

 warmth and cold varies also in sensitiveness in different parts 

 of the body, and we involuntarily test the temperature of an 

 iron or of a poultice by holding it to the cheek, and when we 

 are cold we spread the palms of our hands to the fire. 



If the nerves of the skin are unduly stimulated by severe 

 pressure, or by exposure to extremes of heat or cold, the sense 

 of touch and of temperature is lost in the sense of pain. This 

 is also the case if the nerves are too freely exposed, as when 

 the epidermis is removed by blistering, or in some other man- 

 ner, and the skin is left "raw." 



The sense of taste. The special organ of the sense of taste is 

 the tongue, which is a movable muscular organ covered with 

 mucous membrane. The mucous membrane closely resembles 

 the skin in structure, except that the papillae it contains are 

 more highly developed. The papillae project as minute promi- 

 nences and give the tongue its characteristic rough appearance. 



Some of the papillae are simple and resemble those found in 

 the skin ; the remainder are compound, 1 and are only found on 

 the surface of the tongue. Of these compound papillae there 

 are three varieties. The largest, the circumvallate papillce, are 

 about eight or ten in number, and form a V-shaped row near the 

 root of the tongue, with its open angle turned towards the lips. 



1 A compound papilla is one large one bearing several smaller ones on its sur- 

 face. 



