_Mastication stimulates the heart and so promotes the general circulation._--This stimulating action may be partly due to its local action on the flow of blood and lymph in the jaws and accessory parts, and partly to a reflex influence, but whatever the explanation there can be no doubt of the fact. Hence the mere chewing of a non-nutritive substance, such as gum arabic, is stimulating, and, doubtless, the stimulating effects induced by the chewing of such articles as tobacco and betel are largely to be explained in this way. THE INFLUENCE OF MASTICATION ON THE JAWS AND ADJACENT STRUCTURES This subject is of such importance that it needs to be dealt with in some detail. By “adjacent structures” I mean the masticatory muscles, tongue, teeth, salivary glands, the nasal passages and sinuses pertaining thereto, the naso-pharynx, soft palate, and tonsils. _The muscles of mastication._--Let me at the outset draw attention to certain anatomical points, in connection with the muscles of mastication. These are (_a_) their massiveness; (_b_) the very close relation of the pterygoids to the naso-pharynx; and (_c_) the outward direction of the pterygoids. (_a_) It is not until one studies the muscles of mastication closely that one comes to realise their massiveness. Their large size, in relation to the bony structures in connection with them, is well shown in a vertical transverse section of the head carried through the ascending ramus of the mandible[7] (see Fig. 1). It is evident that the functional activity of so large a mass of muscle tissue cannot but exercise considerable influence on the nutrition of the neighbouring parts. [Illustration: FIGURE 1.--Vertical transverse (slightly oblique) section through the head on a level with the epiglottis. The massiveness of the system of masticatory muscles is apparent.] (_b_) The pterygoid muscles, springing as they do from the internal pterygoid plates, must necessarily be in close relation with the naso-pharynx, especially the internal pair, which take their origin from the internal aspect of the internal plates. I would further point out that the external pair, although they diverge from the naso-pharynx on their way to the mandibular condyles, yet remain on a level with that cavity. This close relation of the pterygoids to the naso-pharynx is, if I mistake not, of great importance in relation to the etiology of “adenoids.”