The Sense of Taste

by Harry L. Hollingworth; Albert T. Poffenberger

The experiments of Parker show further that the mouth and external surface of the body of certain fishes are sensitive to sour, salt, and alkaline solutions. Sheldon obtained about the same results. The external skin covering is not sensitive to sugars. The tongue of fishes presents a smooth, gray, dorsal surface, devoid of elevations or papillæ, which characterize the tongues of many other organisms. Nor is it a mobile organ in comparison with other species. On the whole, the tongue itself seems little adapted for arousing taste sensations.

The system of “lateral line” organs of fishes have at times been thought to be concerned with the chemical sense. This is probably not the case, although their exact function is a question still under dispute.

In a general way, the taste buds, or sense organs of taste of fishes, resemble those of the human being. They are either flask- or cup-shaped, and are composed of two types of cells, called supporting cells and taste cells. The latter cells end peripherally in a hair or bristle, just as the same kind of cell in the human taste bud.

Land-Dwelling Animals

There seems to be no experimental evidence for a specific sense of taste in amphibia, or reptiles. But sense organ structures have been described upon the tongue and soft palate of the frog, where they are said to occur in hundreds. They are disc-shaped structures, made up of several kinds of cells, which correspond to the real taste cells and supporting cells of the human sense organ. The taste cells end peripherally in several hairs or bristles, and at their central end make connection with nerve fibers. In the reptile group there is neither experimental evidence of taste sensitivity nor anatomical evidence of the presence of taste corpuscles on the tongue or in the mouth cavity.

The experimental evidence for the taste sense in birds is slight. It certainly is greatly overshadowed by the keen senses of sight and hearing. Birds seem to represent one case, however, in which taste is more important than smell. Taste sensitivity for different chemical substances, in the case of young chickens, at least, seems clear from certain studies of instinct and learning, in which they accept certain kinds of food and reject others after tasting them. In considering the sense of taste in birds it must be remembered that most of them swallow their food without chewing it or without having it reduced to liquid form through mixture with saliva. The tongue, which varies in character considerably in different types of birds, is in most cases covered with a horny coat. Numerous hard papillæ are found upon its surface. Microscopical examination of these papillæ shows nothing which can correspond to taste buds or to gustatory cells. The parrot is said to form an exception to most birds, in that it has a relatively soft and fleshy tongue, with numerous papillæ, and also in that it chews its food.

In the duck, which has a large tongue, there are certain portions which lack the hard covering common to birds’ tongues. Here, in addition to a large number of tactile corpuscles, there are groups of cells which resemble somewhat true taste corpuscles. The peripheral ends of their cells reach the surface of the mucous covering of the tongue. The cells do not end in the bristle, or hair-like, formation, as those of the human taste cells, but in a pointed elongation of the protoplasm. Experimental evidence of the function of these structures is lacking.

Taste sensitivity and the structure of the taste organs differ greatly in the mammals, but there seem to be two characteristics in common, namely, the localization of the taste corpuscles within the mouth and the importance of the tongue in arousing taste sensations. The character of the mucous lining of the mouth also shows great variation in the number of papillæ and the taste buds which they contain. The number of papillæ varies from two or three in the marsupials and four in the elephant to an extremely large number in rodents, e.g., the rat. The papillæ are in general quite similar to the three most common forms in the human taste organs, the circumvallate, the fungiform, and the filiform, and have about the same location in relation to each other. The greatest difference is in the prominence of the fourth type, the foliate papillæ in certain mammals, as compared with man. These are seen best in the rabbit, as folds directed downward and forward on the sides of the tongue in its posterior portion. They have been considered to result from the great number of papillæ which throw the mucus into folds. Each foliate papilla is composed of a number of parallel ridges, each ridge in turn being composed of papillæ of the fungiform type. Between the ridges there are narrow ditches. It is in the side walls of these that the taste corpuscles are found in greatest numbers. Thus, these ditches are analogous, in function at least, to those of the circumvallate papillæ. Their origin, however, seems to be different from that of the circumvallate. In the monkey one finds less prominent folds on the sides of the tongue, rich in taste corpuscles, which represent the foliate papillæ.

The taste corpuscles themselves have about the same characteristics in all mammals as in man. There are differences in size, to be sure, but their structure is the same, and the supporting cells, gustatory cells, and nerve fibers are present in them all.

This survey of taste in the animal kingdom suggests the conclusion that the taste organs represent a modification of the original skin sensitivity or touch sense, and surely a slight modification when compared with the senses of sight and hearing. A certain resemblance has been remarked by Wundt and others between the touch corpuscles and the gustatory corpuscles. His interpretation is that the whole body was originally endowed with the touch sense, while certain parts being affected continually by specific sorts of stimuli, became adapted to them by undergoing modifications of structure. The head or mouth end of the animal was more subject to chemical stimulation, and the adaptation of the tactile organs to this particular form of stimulation resulted in the development of the senses of taste and smell. To consider taste as one of the lower senses, in the sense of being least highly developed and the earliest to appear, is justified from this survey of the evolution of the taste sense, if from no other point of view.