From these ganglia the branches called the dendrites pass to the sense organ and are found twining around the taste cells in the taste buds. The other branches, the axones, pass into the brain stem and finally end in some brain center. The question, then, is, Do the taste fibers which are found in the lingual branch of the fifth nerve have their cell bodies in the gasserian ganglion, do those found in the ninth nerve have their cell bodies in the petrosal ganglion and those found in the tenth nerve have their cell bodies in the jugular ganglion? These are indeed difficult questions to answer. Reliance must be placed largely upon the results of surgical operations upon these nerves and ganglia, with their resulting effects upon the sense of taste. Earlier reports of surgical operations in which the gasserian ganglion was removed were that the taste sense on the tongue was completely destroyed, suggesting that all of the gustatory fibers of the tongue had their origin in the gasserian ganglion. Later reports, however, were that only the taste sense of the anterior two-thirds of the tongue was destroyed by this operation, suggesting that the fibers found in the glossopharyngeal nerve were independent of the gasserian ganglion and really had their origin in the petrosal ganglion, these fibers getting into the lingual branch of the fifth nerve by a circuitous route.
The experiments of Cushing indicate, further, that operations with removal of the gasserian ganglion produced a dulling or complete loss of the sense of taste in the anterior two-thirds of the tongue, but this loss was followed later by complete recovery. The interpretation of these facts must be that the temporary effect upon the taste sense is only an incidental result of the operation and that the taste fibers for this part of the tongue originate in the geniculate ganglion, which is the ganglion for the facial, or the seventh, cranial nerve, and only get into the lingual branch of the fifth by way of the chorda tympani nerve. Consequently, the chorda tympani, a connecting link between the seventh and the lingual branch of the fifth, is extremely important for the taste sense, since it carries all of the fibers concerned in the taste function of the anterior two-thirds of the tongue. This nerve passes across the cavity of the middle ear, and when stimulated mechanically, chemically, or electrically at this point is said to arouse taste sensations variously described as sour or metallic, sometimes as sweet and bitter. Salt tastes alone have never been reported as resulting from such stimulation.
Fig. 3.—Diagram showing some of the various courses which
have been advocated for the taste fibers in man. (Courtesy W.
B. Saunders Company.)
The taste fibers of the vagus seem to have their cells of origin in the jugular ganglion of that nerve. Thus, while taste fibers are found in the fifth, ninth, and tenth cranial nerves, it seems that the fibers really originate in the seventh, ninth, and tenth nerves. The accompanying figure ([Fig. 3.]), after Cushing, will show the facts about the distribution of the gustatory fibers.
The Cerebral Taste Centers
The first unit in the path connecting sense organ of taste and brain, represented by the neurones having their cell bodies in the ganglia described above, all end in the medulla oblongata, in the neighborhood of the fourth ventricle. These terminals, called the primary sensory nuclei for the seventh, ninth, and tenth nerves, are all included in the nucleus of the solitary bundle.
From these primary sensory nuclei a second series of conducting units begins, sending their fibers in two directions, downward into the spinal cord and upward toward the higher brain centers. Those fibers passing downward make connections in the medulla with the motor centers controlling mastication and swallowing and in the cord with the various motor centers. These connections make possible the reflex responses to taste stimuli, such as secretion of gastric and the other juices of the alimentary canal. Little is known about the course taken by the fibers which must carry the gustatory impulses from the primary sensory nuclei to the higher brain centers. It is quite likely that this second conduction unit is represented by fibers which mingle with those carrying impulses from the sense organs of skin and muscles to the higher centers, and which are found in the median fillet, or the pathway in the brain stem for body sensations. These fibers end in the thalamus.[[7]] In the thalamus a third conducting unit begins and carries the impulses to a still higher center in the cortex of the cerebrum. Exactly what course the fibers take from the thalamus, or just where the cortical center for taste is, has not been definitely determined for human beings.
[7]. The thalamus is a center in the upper part of the brain stem, where connections are made between the cortex or outer layer of the brain proper and lower centers of the nervous system.
Indeed, less is known about the localization of the taste function in the cortex than about any of the other special senses. This is not due to lack of interest, for a large number of researches have been reported in this field. The reason is, rather, that there are certain difficulties in the way of its solution. First, the help to be got from physiological experiments upon animals is limited, because of the close relation between the senses of taste and smell, with the consequent uncertainty in interpreting behavior after surgical operations. And, second, since the exploration of the most accessible portions of the cortex has not revealed a taste center, it is probably located somewhere upon the ventral, or mesial, surfaces of the cerebrum, where experimental work is practically impossible. Clinical and physiological investigations have furnished conflicting results. A survey of recent work suggests that the most probable center for taste is in the hippocampal gyre near the anterior portion of the temporal lobe. No more definite localization is at present possible.