812 FEELING AND TOUCH. 



and middle dorsal and under surface of the tongue, the front portion of the 

 hard palate, the posterior pillars of the fauces, the gums, and the lips. 

 Sapid substances are unsuitable as a test for this purpose, on account of their 

 rapid diffusion. Bitter substances produce most effect when placed on the 

 back, and sweet substances when placed on the tip, of the tongue ; but the 

 tasting power of the tip of the tongue varies very much in different indi- 

 viduals, and in many seems almost entirely absent. It is said that acids are 

 best appreciated by the edge of the tongue. 



735. It is essential for the development of taste that the substance to 

 be tasted should be dissolved, and the effect is increased by friction. The 

 larger the surface the more intense the sensation. The sensation takes some 

 time to develop, and endures for a long time, though this may be in part 

 due to the stimulus remaining in contact with the terminal organs. A tem- 

 perature of about 40 is the one most favorable for the production of the 

 sensation. At temperatures much above or below this, taste is much im- 

 paired. The nerves of taste are, as we have said, the glosso-pharyngeal 

 and the lingual or gustatory. The former supplies the back of the tongue, 

 and section of it destroys taste in that region. The latter is distributed to 

 the front of the tongue, and section of it similarly deprives the tip of the 

 tongue of taste. There is no reason for doubting that the gustatory fibres 

 in the glosso-pharyngeal are proper fibres of that nerve ; but it has been 

 urged by many that the gustatory fibres of the lingual are derived from the 

 chorda tympani, and that those fibres of the lingual which come from the 

 fifth are employed exclusively in the sensations of touch and feeling ; the 

 evidence in favor of this view is, however, inconclusive. 



CHAPTER Y. 



FEELING AND TOUCH. 



GENERAL SENSIBILITY AND TACTILE PERCEPTIONS. 



736. WE have taken the foregoing senses first in the order of discussion 

 on account of their being eminently specific. The eye gives us only visual 

 sensations, the ear only auditory snsations. The sensations are produced in 

 each case by specific stimuli ; the eye is only affected by light and the ear 

 only by sound. Moreover, the information they afford us is confined to the 

 external world ; they tell us nothing about ourselves. The various visual 

 sensations which arise in our retina are referred by us not to the retina itself, 

 but to some real or imaginary object in the world without (including as part 

 of the external world such portions of our own bodies as are visible to our- 

 selves). Such also, with diminishing precision, is the information gained by 

 hearing, taste, and smell. 



All the other afferent nerves of the body, centripetal impulses along which 

 are able to affect our consciousness, are the means of conveying to us in- 

 formation concerning ourselves. The sensations, arising in them from the 

 action of various stimuli, are referred by us to appropriate parts of our own 

 body. When any body comes in contact with our finger, we know that it is 

 our finger which has been touched ; from the resultant sensations we not only 

 learn the existence of certain qualities in the object touched, but we also are 



