494 LECTURE XXI. 



the soft and hard palate, the anterior pillar, the tonsils, the posterior pillar, 

 the uvula, the epiglottis, and even the throat, itself. This wide distribution 

 of the organs for taste perception is only noticeable during youth. In 

 adults the sensation is more localized, although it varies greatly with indi- 

 viduals. The mucous membrane of the cheeks, the uvula, tonsils, and 

 the middle of the tongue, are almost always incapable of taste perception 

 in adults. The end-apparatus of the taste-nerves form the so-called taste- 

 buds or taste-goblets. In man the glossopharyngeal and trigeminal nerves 

 have been recognized as taste-nerves. The former innervate the back 

 part of the tongue, the latter the front part. Individual peculiarities are 

 noticed here, and sometimes one of these nerves alone provides for the whole 

 region. The different sensations of taste may in general be attributed 

 to four different qualities; namely, sweet, sour, bitter, and salt. We also 

 speak of alkaline and metallic tastes. There is hardly room for doubt that 

 these different sensations of taste are produced by the aid of different 

 nerves, so that for the taste-nerves the law of specific sense energy * applies. 

 According to this law, one and the same excitant when acting upon different 

 nerves will always produce different sensations, while, on the other hand, 

 all sorts of different excitants always produce the same sensation when 

 acting upon the same nerve. The different quality of the sensation is 

 therefore merely caused by the different nature of the end-apparatus in 

 the central nervous system. Moreover, the peripheral reception appara- 

 tus is so specialized that it is only affected by certain definite excitants. 

 It might be thought, and especially in the study of the sensation of taste, 

 that the chemical constitution of a substance having a definite quality of 

 taste might give us some idea of the way in which the definite end-apparatus 

 is excited. Numerous experiments have been performed with this idea 

 in mind, but without success. 2 Thus, for example, a great many amino 

 acids taste sweet, while others are bitter. Glycocoll, alanine, and a-amino- 

 valeric acid are sweet, while Z-leucine, which occurs in nature, is bitter. 

 It is remarkable that d-leucine has a sweet taste. 3 In dWeucine the sweet 

 taste predominates. The sense of smell is closely related to that of taste, 

 and frequently they are confused with one another. It is, furthermore, 

 interesting that both these sensations of taste and of smell may be pro- 

 duced by means of a very small amount of material. The organ of smell 

 is in some cases especially sensitive. Emil Fischer and Penzoldt found, 

 for example, that as little as 0.000,000,04 milligram of mercaptan in a 



1 Johannes Muller: Zur vergleichenden Physiologie des Gesichtssinnes. Leipzig 

 (1826). Cf. R. Weinmann: Die Lehre von den specifischen Sinnesenergien, Hamburg 

 and Leipzig (1895). 



2 Cf. Wilhelm Sternberg: Arch. Anat. Phys. 1903, 538; 1904, 483; 1906, 201; Ber. 

 15, 36 (1905). 



3 Emil Fischer and Otto Warburg: Ber. 35, 3997, 4005 (1905). 



