We may well pause here and consider briefly some of these processes which play so prominent a part in the proper utilization of the three classes of organic foodstuffs. The first digestive fluid which the ingested food comes in contact with is the saliva. Sensory nerve fibres, chiefly of the glossopharyngeal and lingual nerves which supply the mouth and tongue, are stimulated by the sapid substances of the food, and likewise by mere contact of the food particles with the mucous membrane lining the mouth cavity as the food is masticated and rolled about prior to deglutition. Impulses communicated in this way to the above sensory nerves are transmitted to certain nerve centres in the medulla oblongata, whence impulses are reflected back through secretory nerves going to the individual salivary glands, thereby calling forth a secretion. The production of saliva is thus a simple reflex act, in which the food consumed serves as a true stimulant or excitant. Pawlow,[5] indeed, claims a certain degree of adaptability of the secretion to the character of the food taken into the mouth. Thus, he finds that dry, solid food excites a large flow of saliva, such as would be needed to masticate it properly and bring it into a suitable condition for swallowing. On the other hand, foods containing an abundance of water cause only a scanty flow of saliva. The situation of this secretory centre in the medulla, and the many branchings of nerve cells in this locality would naturally suggest the possibility of salivary secretion being incited by stimuli from a variety of sources. This is indeed the case, and it is worthy of note that a flow of saliva may result from stimulation of the sensory fibres of the vagus nerves as well as of the splanchnic and sciatic, thus indicating how a given secreting gland may be called into activity by impulses or stimuli which come to the centre through very indirect and devious pathways. Further, the secretory centre may be stimulated, and likewise inhibited, by impulses which have their origin in higher nerve centres in the brain. These facts are of great importance in throwing light upon the ways in which a secretion like saliva is called forth and its digestive action thus made possible. The thought and the odor of savory food cause the mouth to water, the flow of saliva so incited being the result of psychical stimulation. Similarly, fear, embarrassment, and anxiety frequently cause a dry mouth and parched throat through inhibition of the secretory centre by impulses which have their origin in higher centres in the brain. [5] Pawlow: The Work of the Digestive Glands. Translated by Thompson. London, 1902.