NERVES OF THE TONGUE. 429 



The organ of taste is placed at the commencement of the digestive ca- 

 nal; hence the characters of substances may be examined usesofthe 

 with deliberation while they are yet under the control of the sense of taste - 

 will, for when once a body has entered the oesophagus it is swallowed in- 

 voluntarily. The tongue, therefore, gives warning of the presence of del- 

 eterious substances, and in no small degree excites the appetite by receiv- 

 ing the impression of pleasant flavors. The essential condition under 

 which it acts is a moist state of its surface, for the dry tongue, though it 

 enjoys common sensibility, after the manner of any portion of the exter- 

 nal tegument, does not enjoy taste. One of the duties of the salivary 

 glands is incidentally to maintain this moistened condition. To a cer- 

 tain degree, taste may be regarded as a refinement on touch. It differs 

 from vision and hearing in the peculiarity that there is no sin- serves of the 

 gle nerve of special sense individually devoted to it, for the tongue, 

 front of the tongue is supplied by the lingual branch of the fifth pair, 

 and the back by the glosso-pharyngeal. Its entire nervous supply is 

 derived from four different sources: the lingual, the hypoglossal, the 

 glosso-pharyngeal, and the sympathetic, representing therefore special 

 sensibility, muscular motion, common sensibility, and sympathetic rela- 

 tion. That the hypoglossal is the nerve of motion, or muscular nerve, is 

 proved beyond doubt by its section, after which the motions of the tongue 

 are destroyed, but taste. and touch remain. The individual duty dis- 

 charged by the glosso-pharyngeal, and the lingual branch of the fifth pair 

 respectively, is not so clearly determined. Section of the former is at- 

 tended with loss of taste, though it is not yet proved that there is a loss 

 of all kinds of taste. If the lingual branch of the fifth be divided, com- 

 mon sensation at the tip of the tongue is destroyed, and there is evidence 

 that with this the appreciation of certain tastes disappears. The glosso- 

 pharyngeal is distributed to the circumvallate papillae, and it is said that 

 in some birds the lingual is suppressed. Upon the whole, therefore, it 

 may be concluded that these nerves are conjointly engaged in the sense 

 of taste, the glosso-pharyngeal being engaged with those flavors which 

 affect the back part of the tongue, the lingual with those which affect 

 the tip. 



Illustrations of the distribution of the hypoglossal nerve have already 

 been given in its description, under the title of the twelfth pair. 



The surface of the tongue presents the tactile and gustative powers in 

 an inverse manner. Examined by the method described in Tactile and 

 the chapter on touch, the compasses must be opened to a great l^* 1 ^ the" 

 extent, as we pass from the tip toward the back of the tongue, tongue. 

 in order that a double impression may be perceived. This condition ap- 

 pears to be in accordance with the requirements of the organ, common 

 tactile sensibility being most necessary at its outer extremity, and this 



