Papillae and Nerves of the Tongiie of the Frog. 287 



the currants and leaving the principal stalk with their project- 

 ing pedicels. 



Physiologicaluctio7iofthe Tongue. — Thefungi former neuro- 

 vascular papillae are the parts which more especially deserve 

 our attention, as their structural arrangement points them out 

 as being evidently the principal if not the sole organs of taste 

 in the tongue. Considered under the simplest point of view, 

 they may be regarded as hollow vesicles, containing a coil of 

 nerves and blood-vessels within them, and when a liquid is 

 brought in contact with its external membrane, its thinness 

 causes it to be rapidly permeated by the sapid substance whicii 

 then comes in immediate contact with the nerve where the 

 sensation of taste is created. The capillary coil then by its 

 internal current and its extensive surface, rapidly carries it 

 away and leaves the nerve free to receive a fresh impression. In 

 analysing this mechanism, we may for convenience distinguish 

 three separate periods or stages. In the first the body per- 

 meates the vesicular membrane ; in the second it comes in con- 

 tact with the nerve and produces an impression which is con- 

 veyed to the brain ; in the third this substance is eliminated 

 from the cavity and from the nerve. The first is a well-known 

 property of all animal membranes, of allowing themselves to 

 be traversed by the various liquid and gaseous substances with 

 which they are in contact. We have an instance of this when 

 venous blood contained in a bladder and exposed to the air or 

 immersed in water containing air in solution, becomes crimson 

 from the passage of the oxygen through the membrane into 

 the blood. A similar action takes place when various other 

 liquiils and gases are separated from each other by an organic 

 membrane. Acetic acid applied to the tongue of the frog over a 

 blood-vessel fiUetl with stagnating blood, destroys its red colour 

 and dissolves the tunic of the vesicles in the same way as when it 

 is applied to blood taken from the vessels. A still more notable 

 example is presented bythe branchiaeof the tadpole, which may 

 be observed during life. There the blood is perceived circulating 

 with great rapidity in the capillary vessels, and separated from 

 the surrounding water by a very thin membrane. Nevertheless 

 in virtue of the permeability of this membrane, a constant cur- 

 rent exists which causes the oxygen in the water to be absorbed 

 by the blood, and the carbonic acid of the blood to pass into 

 the water. In this manner the respiration of the animal is 

 effected. The rate at which a membrane is traversed by a 

 li(|ui<l is ceteris pari/ms in proportion to the thinness of the 

 membrane. At the branchial surface it is extremely thin, on 

 accomit of the distinctness with which the globules of the blood 

 appear tiiruugh it. For the same reason, it is evident that the 



