The next step is swallowing. When the food is sufficiently triturated and moistened, it is collected together by means of the tongue and placed upon the upper surface of that organ, which becomes concave and presses or rolls the soft pulp against the hard palate so as to shape it into an -oblong mass or 10 bolus (Fig. 51). The apex of the tongue is now raised and pressed against the hard palate, and by the successive elevations of the different parts of the dorsum of the tongue the bolus is gradually pushed backward toward the isthmus of the fauces.

Section through a portion of Dentine next the pulp cavity of a growing tooth.

Fig. 49. Section through a portion of Dentine next the pulp cavity of a growing tooth. (a) An isolated odontoblast; {b) Growing part; (c) Odontoblasts; (d) Filaments of protoplasm projecting from the tubules of hard dentine. (Beale).

The Pterygoid Muscles seen from without after removal of the superficial parts.

Fig. 50. The Pterygoid Muscles seen from without after removal of the superficial parts, the temporal muscle, the zygomatic arch, and a portion of the lower jaw and masseter. (1) External, (2) Internal pterygoid muscle.

The root of the tongue with the hyoid bone is at the same time drawn upward and forward, so that the bolus easily slips down along the retreating slope leading from the mouth cavity, and gets within the reach of the constrictors of the fauces. Immediately before the morsel of food is grasped by the muscles of the fauces, the levator palati draws the soft palate upward and backward to completely close the posterior openings of the nasal cavity, as is shown by the fact that during the act of swallowing the pressure in the nasal cavity is raised. At the same moment the intrinsic muscles of the larynx, which surround the rima glottidis like a constrictor, firmly close that opening by approximating the cords and arytenoid cartilages. The entire larynx is at the same time drawn up behind the hyoid bone by the thyro-hyoid muscle. The rima glottidis is thus tucked in under the cushion of epiglottis, while the leaf of the epiglottis is pulled down over the larynx by the oblique aryteno-epiglottidean and thyro-epiglottidean muscles.

While the closure of the nasal and pulmonary air passages is going on, the bolus has passed out of the cavity of the mouth and has been caught by the palato-glossal and palatopharyngeal muscles, which force it into the pharynx and at the same time close the isthmus faucium behind the descending morsel. The stylo-pharyngeis and the pharyngeal constrictors now grasp the bolus spasmodically, and the latter contract in rapid succession, moving the bolus onward, and drawing themselves over it, pass it on to the oesophagus, where, by a progressing ring-like contraction of the circular muscles and a simultaneous shortening of the longitudinal layer of fibres, the mass is slowly squeezed down to the cardiac orifice of the stomach. The movements of the oesophagus are essentially peristaltic in character, the peculiarities of which form of motion will be discussed when speaking of the intestinal movements.

Muscles of Tongue and Pharynx.

Fig. 51. Muscles of Tongue and Pharynx.

1,2,3, Muscles from styloid process (b) to the tongue, hyoid bone (d) and pharynx respectively; 4, 5, 6, 7, 8, muscles of tongue; 9, 10, is, constrictors of pharynx; 12, oesophagus; 13, is placed on larynx (e). (Allen Thomson).

The process of swallowing is performed by a continuous series of coordinated muscular movements, quite independent of gravitation, as may be seen in animals drinking with their heads downward. Although these complex sets of movements follow each other regularly and without any check or interval, the act of deglutition is commonly divided into three stages, between which, as there is no pause, it is not easy to draw a hard and fast line.

Deep Muscles of Cheek, Pharynx, etc.

Fig. 52. Deep Muscles of Cheek, Pharynx, etc.

(1) Orbicularis oris; (2) buccinator; (3) superior, (4) middle and (5) inferior constrictors of the pharynx; (6) oesophagus; (7) styloid muscles cut across; (8,9, 10) muscles attached to the hyoid bone (d) and thyroid cartilage (e). {Allen Thomson).

The first stage is simply the initiatory step of placing the morsel of food or some liquid in such a position as to excite the second or spasmodic act of deglutition. This first step is a voluntary act, and it is the only part of the movements of swallowing over which we can exert complete control. The progress of the morsel between the tongue and palate toward the fauces may be as slow and gradual as we wish, but the moment a certain point is reached volition is at an end, and we are unable to check the completion of the act.

By the second stage is meant the period occupied by the passage of the food bolus through the pharynx and past the top of the larynx. Although we are not able to influence it in any way by our will, we are conscious of the food passing in this region. It is a rapid, involuntary spasm in which a great number of muscles take part, all of which are made up of striated muscle tissue.

The third stage includes all the rest of the time during which the bolus is passing from the grasp of the lower pharyngeal constrictor and along the oesophagus. Not only has our will no influence over this stage of deglutition, but we are hardly conscious of its taking place, since no sensations accompany the greater part of it. Thus the more essential movements of the act of swallowing are purely reflex and involuntary, though we can call forth this series of reflexions by voluntary stimulation of a certain part of the fauces by means of a morsel of food or a drop of liquid, and without such a stimulus as food or liquid we cannot by our will excite swallowing. We think we can perform the muscular movements of swallowing when we please, without any food or fluid, but in this we are mistaken, as careful observation of our own performance of the act will show.

Transverse section of Oesophagus. {Horsley).

Fig. 53. Transverse section of Oesophagus. {Horsley).

a. Outer fibrous covering.

b. Bundles of longitudinal muscle cut across.

c. Transverse muscular coat cut obliquely.

d. Sub-mucous coat with glands in section.

e. Muscular layer of the mucous membrane. /. Mucous membrane with cut vessels.

g. Stratified epithelium.

The pharyngeal spasm is always preceded by the deposition in the region of the isthmus faucium of some drop of saliva collected from the mouth or fauces themselves. In fact, without a slight preliminary movement of the posterior part of the tongue - which might be called the last act of mastication - the more essential stages of deglutition cannot be excited.