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. Footnote 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. ------------------------------------------------------------------------ CHAPTER VII TASTE-PRODUCING SUBSTANCES _Adequate and Inadequate Stimuli_