Chap. XX] INTERNAL AND EXTERNAL SENSES 411 



by passing a metallic point slowly over the skin. At certain points 

 a feeling of contact or pressure will be experienced, and at other 

 points a feeling of cold or heat, depending on whether the tempera- 

 ture of the instrument is higher or lower than that of the skin. 



TASTE 



Necessary conditions. — Aside from the conditions which are 

 always necessary for sense-perception, — viz. proper organs for 

 receiving, communicating, and perceiving the sensory impulse, — 

 there must be present a sapid substance which must be in solu- 

 tion. The solution in the case of dry substances is effected by 

 saliva. It is also necessary that the surface of the organs of taste 

 shall be moist. The substances which excite the special sensa- 

 tion of " taste " act by producing a change in the terminal fila- 

 ments of the gustatory nerve (branch of the glossopharyngeal) 

 and this change furnishes to it the required stimulant. 



Organs of taste. — The special organs of the sense of taste are 

 end organs of nerve filaments w'hich are derived from the seventh 

 and ninth cranial nerves. These end organs are called taste buds 

 and are situated chiefly on the surface of the tongue, though there 

 are some of these organs scattered over the soft palate, fauces, 

 tonsils, and pharynx. 



The tongue. — The tongue is a freely movable muscular organ 

 consisting of two distinct halves united in the centre. The base 

 or root of the tongue is directed backward and is attached to the 

 hyoid bone by numerous muscles. It is connected with the epi- 

 glottis by three folds of mucous membrane, and with the soft 

 palate by means of the anterior pillars of the fauces. 



PapillcB of the tongue. — The tongue is covered and lined with 

 mucous membrane. The mucous membrane on the under surface 

 is similar to that lining the rest of the mouth, but the mucous 

 membrane on the upper surface is studded with papillae which pro- 

 ject as minute prominences and give the tongue its characteristic 

 rough appearance. Of these papillae there are three varieties : — 



(1) Circum vallate (walled in) papillae are the largest, are circu- 

 lar in shape, and form a V-shaped row near the root of the tongue, 

 with its open angle turned toward the lips. They serve to secrete 

 mucus and contain taste buds in which the filaments of the glosso- 

 pharj^ngeal nerve terminate. 



