- The voluntary influences which regulate the motions of the muscles of mastication pass along the efferent branches of the fifth nerves (trigemini) which accompany its inferior division. The muscles which depress the jaw to open the teeth and the intrinsic muscles of the tongue are supplied by the ninth pair of nerves (except the posterior belly of the digastric, which has a branch from the facial, and the mylohyoid and anterior belly of the digastric, which are supplied from the third division of the fifth). The coordination of the movements of mastication and suction seem to reside in the medulla oblongata, but are obviously under the control of the will.
The afferent impulses which excite the nerve centres in the medulla, and give rise to reflex acts which cause the swallowing movements, pass from the mucous membrane of the fauces along (i) the descending palatine branches of the spheno-palatine ganglion and the second division of the trigeminus, also along (2) the pharyngeal branches of the superior laryngeal branch of the vagus to the medulla, where the coordination of pharyngeal spasm and oesophageal peristalsis is accomplished. Thence the efferent impulses pass by (1) the hypoglossal to the hyoid and glossal muscles, (2) the glosso-pharyngeal and vagus to the pharyngeal plexus to supply the constrictors, and (3) the facial and fifth to supply the fauces and palate, as indicated by their anatomical distribution.
The act of deglutition can be readily excited in an animal which is deprived of all the nerve centres down to the medulla oblongata, and may also be seen in those human monstrosities (anencephalous foetus) without the upper part of the brain being developed, but which can notwithstanding both suck and swallow.
The movements of the oesophagus are reflections from the central nervous system (medulla), both sets of impulses (possibly the afferent and certainly the efferent) passing along the branches of the vagus.
It would appear that the normal peristaltic movements of the oesophagus are always initiated by a pharyngeal spasm, and that they form an inseparable sequel to it. Thus the wave of contraction passes along the entire length of the oesophagus even when the bolus is stopped mechanically, and, on the other hand, a body introduced into the oesophagus without passing through the pharynx excites no peristaltic wave, and remains motionless.
But it has been observed, in apparent contradiction to the foregoing statement, that the oesophagus when removed from the body, and therefore quite independent of the pharynx and its nervous connections, can be excited to move peristaltically. In this case the medulla or vagus can have no part in bringing about this wave of movement. To explain this discrepancy, it may be urged that the local nerve and muscle mechanism in the tissues of the oesophagus are capable by themselves of carrying out peristaltic contraction independently of the central nerve organs, but that this power is, under ordinary circumstances, held in check by the vagus. The inhibition is temporarily suspended as a sequence of pharyngeal spasm, and consequently a wave of peristaltic contraction is excited in the oesophageal muscles, either in response to the direct stimulus of a passing bolus, or as a result of impulses reflected along the vagus channels from the medulla.
Fig. 54. Diagram of Wall of the Stomach, showing the relative thickness of the mucous membrane (a, b, c) and the transverse (e), oblique (/) and longitudinal muscle fibres.