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TEXT-BOOK OF PHYSIOLOGY. 



tions of contact and muscle sensations. The union of these sensa- 

 tions forms the basis of the perception of hardness, softness, smooth- 

 ness, and roughness of bodies. 



THE SENSE OF TASTE. 



The physiologic mechanism involved in the sense of taste includes 

 the tongue, the gustatory nerves (the chorda tympani and the glosso- 

 pharyngeal), their cortical connections and nerve-cells in the gray 

 matter of the fourth temporal convolutions. The peripheral 

 excitation of this apparatus gives rise to nerve impulses which 

 transmitted to the brain evoke the sensations of taste. The 

 specific physiologic stimulus is matter, organic and inorganic, in a 

 state of solution. 



The Tongue. The tongue consists of both intrinsic and extrinsic 

 muscles, in virtue of which it is susceptible 

 of both a change in shape and position. 

 The movements of the tongue, though not 

 essential to taste, are made use of in the 

 finer discrimination of tastes. 



The tongue is covered over by mucous 

 membrane continuous with that lining the 

 oral cavity. The dorsum of the tongue 

 presents a series of papillae richly supplied 

 with blood-vessels and nerves. Of these 

 there are three varieties, the filiform, the 

 fungiform, and the circumvallate (Fig. 275). 



1. The filiform papilla, the most numerous, 



cover the anterior two-thirds of the 

 tongue; they are conical or filiform in 

 shape and covered with horny epithe- 

 lium which is often prolonged into 

 filamentous tufts. 



2. The jungijorm papilla, found chiefly at 



the tip and sides of the tongue, are 

 less numerous but larger than the pre- 

 ceding and of a deep red color. 

 3. The circumvallate papilla, from eight to ten in number, are 

 situated at the base of the tongue arranged in the form of the 

 letter V. They consist of a central projection surrounded by 

 a wall or circumvallation from which they take their name. 

 The Peripheral End-organs. The Taste-buds. Embedded 

 in the epithelium covering the mucous membrane not only of the 

 tongue but of the palate and posterior surface of the epiglottis are 

 small ovoid bodies which from their relation to the gustatory nerves 



FIG. 275. THE TONGUE. 

 i . Papillae circumval- 

 latae. 2. Papillae fungi- 

 formes. 



