Papillae and Nerves of the Tongue of the Frog. 289 



mulate around the peripheric extremities of the nerve, like 

 a chemical substance ; it is therefore a matter of no surprise 

 that these nerves are not surrounded by so rich a vascular ex- 

 pansion. Nay, further, with respect to the action of caloric, 

 we are aware that in virtue of its conductibility it tends quickly 

 to attain a state of equilibrium, which in extreme cases may 

 be in a degree hastened by the vascular circulation of a fluid 

 of an equal temperature like the blood, which if developed to 

 the same extent as in the fungiform papilla, would, it is ra- 

 tional to suppose, so much equalize the distribution of caloric 

 as to render the nerve incapable of distinguishing minute dif- 

 ferences of temperature. But without entering any further 

 into these speculations out of the pale of experimental science, 

 it is proper to state that numerous facts derived from phy- 

 siology and pathology are in favour of what I have deduced 

 from the examination of the anatomical structure of the two 

 speciesof papillae, namely, theexistence of two distinctspeciesor 

 sets of organs in the tongue, those for appreciating the chemical 

 properties of bodies, and those for taking cognizance of their 

 physical and mechanical attributes. In numerous instances 

 the faculty of taste is entirely destroyed, leaving the tactile 

 properties unaffected. 



With regard to the second period or stage, when the blood 

 comes in contact with the nerve and produces the impression 

 which is conveyed to the brain, we can form no idea of the 

 various actions which are produced within the nerve; we can 

 only surmise that these impressions may either resemble elec- 

 tric currents, such as arise from the contact of two heteroge- 

 neous substances, or that they are caused by a kind of mole- 

 cular perturbation, which is transmitted upwards to the brain, 

 where it produces a kind of image of a more permanent nature. 

 We know Uiat different liquids vary greatly in their power of 

 traversing the same membrane ; alcohol and water, for in- 

 stance, in a bladder become separated from each other by the 

 water traversing the membrane and the spirit remaining be- 

 hind. To this cause may perhaps in some small degree be 

 attributed the difference in the sensations which different sub- 

 stances give rise to ; but by far the most efficient cause is 

 probably the peculiar manner in which the nerve is affected, 

 as different rays of light excite different sensations indepen- 

 dently of the intensity of their action. My experiments have 

 not enabled me to ascertain whether the nervous fibrils sup- 

 plying the two kinds of papilla.' are the same or of different 

 origin, or whether they are rendered distinct in their actions 

 merely by means of the structure of the jjarts around them. 



Tlie third stage may be considered as simply a kind of per- 

 Phil. Mag. S. 3. Vol. 30. No. 20 1 . yJpril 1 84.7. X 



