(_c_) Of the two pairs of pterygoids the external pair pass in the more outward direction, forming with the sagittal plane of the head an angle of 45° (see Figs. 1 and 2). In consequence of this direction they tend by their contraction to pull the pterygoid plates and posterior parts of the maxilla away from the sagittal plane of the head, and thus to secure the normal width of the posterior nares. It is these muscles which bring about the lateral movements of the mandible, causing the lower teeth to move laterally and sagittally across the upper, the food being in this way far more effectually ground than by a mere vertical pressure of the teeth against one another. These lateral movements are, as we shall see, less pronounced among the moderns than among primitive peoples. [Illustration: FIGURE 2.--Portion of horizontal section of head about an inch below the condyles of the lower jaw. The outward direction of the external pterygoids is well shown; also the close relation of the levatores and tensores palati with the internal pterygoids.] _The influence of the contraction of the masticatory muscles on the local circulation of blood and lymph._--When a muscle is at rest the blood flows sluggishly through it, while there is a complete, or all but complete, stagnation of the lymph current; if a lymphatic trunk of a limb at rest be cut no lymph escapes from it. Rhythmic muscle contractions, however, stimulate the flow both of blood and lymph (_a_), in the contracting muscles themselves and (_b_) in the neighbouring parts. (_a_) Not only are the muscle arteries dilated during rhythmic contractions, but the blood is vigorously squirted out of the muscle veins, so that much more blood flows through a muscle during its rhythmic contraction than during rest. The flow of lymph is even more markedly stimulated,--this fluid, which, while the muscle is at rest, is stagnant or all but so, being during contraction driven actively along the lymphatic trunks. (_b_) How greatly rhythmic muscle contractions influence the circulation of fluids in the neighbouring parts is shown by the flushing of the skin and the swelling of the soft parts generally of a limb which is being exercised. We thus see how profoundly the exercise of the masticatory muscles--and among these we must not forget to include the tongue--influences not only their own nutrition but that of the important structures adjacent to them--that is to say, of the jaw-bones, salivary glands, buccal mucous membrane, soft palate, faucial tonsils, pharynx, and naso-pharynx, as well as of the nasal cavities and their accessory sinuses. All these parts are during mastication copiously flushed with blood and lymph, from which it is evident that efficient mastication must stimulate their nutrition and favour their proper development. Hence, in one who has from childhood upwards been accustomed to masticate efficiently, we generally find these parts well developed, the jaws large and shapely,