144 PHYSIOLOGY CHAP. 



that they become altered and decomposed on contact with the 

 buccal secretions, the difference in the two classes of substances 

 nevertheless indicates that to be capable of stimulating the organs 

 of taste, all substances must be in a state of true and perfect 

 solution ; colloids only form pseudo-solutions, and exist in fluids 

 in the form of groups of several molecules. As all food-stuffs 

 except the sugars are insoluble in water, and therefore quite 

 devoid of taste, it follows that the gustatory characters by which 

 we are able to recognise food-stuffs in general (proteins, colloidal 

 starches, fats) depend on the minute traces of soluble crystalloid 

 and volatile substances mixed with them. 



In order that the substances dissolved in water may come into 

 contact with the nerve-endings of the taste-cells, it is necessary 

 that they should first diffuse in the fluid contained in the pores 

 of the taste-buds ; this causes a certain delay in their chemical 

 action, and possibly a certain temporary separation of the active 

 components of the solution, if the rate of diffusion differs. 



The taste papillae are so arranged that all substances intro- 

 duced into the oral cavity in sufficient amount must come into 

 contact with them. The roughness of the lingual surface owing 

 to the projection of the papillae, and the numerous interpapillary 

 depressions, are conditions that promote the retention of the solid 

 and liquid particles in the interpapillary spaces and the diffusion 

 of the dissolved substances to the gustatory pores. 



The movements of the tongue increase the surface of contact 

 between the organ of taste and the contents of the mouth. Pick 

 held that these movements increased the excitability of the 

 gustatory nerve-endings by mechanically stimulating the organs 

 of taste. He went so far as to declare that the substances applied 

 to the dorsum of the tongue when it is stationary are scarcely 

 perceived or not at all. But the subsequent researches of Oehrwall 

 and others disproved this statement and showed that the lingual 

 movements merely increase the surface of contact between the 

 active substance and the sense organs. At most it may be said 

 that the pressure of the tongue against the hard palate facilitates 

 the penetration of the fluids into the interpapillary spaces. 



The salivation of the bolus, for reasons that can readily be 

 appreciated, facilitates the solution, diffusion, and perception of 

 the flavours of solid and semi-fluid substances. The secretion of 

 the salivary glands and of serous glands that open into the fossae 

 of the circumvallate papillae also exert a protective function upon 

 the gustatory organs, because in the case of unduly strong tastes 

 there is a reflex stream of buccal secretion which diminishes their 

 action on the specific nerve-endings. 



Haller assumed an erection of the lingual papillae in gustatory 

 activity, but this is disproved by the observations of Bidder and 

 Oehrwall. 



