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elements of the food received, and to lubricate their passage along the tongue, and the isthmus of the fauces.

These distinctions, which appear so ingenious and natural at first sight, Schiff has shown to be, perhaps, a little too sharply defined; thus, mastication alone, that is, not accompanied by any gustatory impression, has little or no influence on the parotid secretion: in the case of all the salivary glands, the impression of taste, joined to the masticatory movements, are the most powerful means of producing secrétion.

The quantity of saliva secreted in a day has been variously estimated on account of the intermittent form of secretion. In dogs, it is as much as 1500 grammes. This secretion though more especially sensible during mastication, however, is continuous; because the saliva is necessary to keep the mouth moist, to assist the movements of the tongue (speech), and, as we have already said, for the purpose of deglutition. We shall find that, by means of the saliva, movements of deglutition are produced from time to time and at very short intervals, the purpose of which is to preserve the function of the organ of hearing.

C. Deglutition.

When the food has been so mixed with the saliva as to become capable of movements like a fluid, it is subjected to a pressure, which forces it downwards, from the buccal cavity to the cardiac orifice of the stomach; in other words, it leaves the mouth, and passes through the pharyngeal and œsophageal tubes. The principle governing the movement of the food is the same as that which governs the movement of fluids, that is, excessive pressure at one point and none at all at others, thus destroying the equilibrium of the fluid, and causing it to flow in the direction in which the pressure is slightest. This principle applies to the deglutition of solids, the state of semi-liquefaction into which they are brought imparting to them mechanical properties similar to those of fluids.

The organs of deglutition consist (Fig. 62), first of the buccal cavity,-bounded, above, by the roof of the mouth; at the back, by the velum of the palate; below, by the tongue; and, in front, by the teeth. After the buccal cavity, we come to the pharynx, at the level of which the alimentary canal. communicates with the windpipe; or, rather the two passages cross each other (communication from above and behind with the nasal chambers-the first part of the windpipe;

below, and in front, with the larynx- the continuation of the wind-pipe). A most important feature in deglutition is the

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mechanism by which the upper and the lower orifices of communication become obliterated.

When mastication and insalivation are completed, the food collects in a single mass on the surface of the tongue, the tip of which touches the roof of the mouth while the

*k, h, Buccal aperture. 1, Tongue. d, Lower jaw, with the genio-glossal inserted. e, Hyoid bone. y, Epiglottis. f, Cavity of the larynx (with the opening of the ventricles). c, Velum palati. u, Anterior pillar of the uvula. v, Posterior pillar. t, Tonsil. 8, Narrow part of the pharynx, connected with the oesophagus. z, Orifice of the Eustachian tube, at the upper part of the pharynx.

food passes downwards to its base. As the food reaches the front pillars of the velum of the palate - being pushed into the pharynx by the tongue laid against the roof of the mouthit is seized by the pharynx, which rises before it, on account of the contraction of its longitudinal fibres. The circular fibres of this muscular tube immediately contract successively, and drive the food before them into the œsophagus, where it continues its progress by means of a similar peristaltism that is, successive contraction of the circular muscular fibres, driving the food before them, while the contraction of the longitudinal fibres draws towards it those parts of the tube in which it is to become involved.

While the food crosses the pharynx, the two communications between this tube and the windpipe are obliterated.

The upper communication (pharynx and nasal chambers) is not obliterated by a movement of the velum of the palate resembling that of a drawbridge, as was for a long time supposed (Bichat); it takes place by means of the posterior pillars of the velum of the palate. In order to effect this obliteration, the pillars approach each other: while the muscular fibres of these pillars (pharyngeal muscles) are directed

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Fig. 63. Diagram, showing the occlusion of the naso-pharyngeal passage, by the action of the muscles of the posterior columns (Staphylo-pharyngeals.)*

obliquely, downwards and backwards, across the lateral walls of the pharynx, and are again joined together, along a considerable part of the posterior median line, so as to form an

*A, This part is seen in profile. N, Nasal cavity. B, Mouth. L, Tongue. E, Epiglottis. 7, Uvula. P, P, Course of the staphylo-pharyngeal muscle.

B, Diagram of the orifice, enclosed by the two staphylo-pharyngeals as by a sphincter. 1 (P), In the state of repose. 2 (P), Semi-occlusion. 3 (P), Perfect occlusion. 1, Uvula.

elliptic sphincter, in an oblique line from front to back and from top to bottom. (Fig. 63.) The anterior and posterior extremities of this elliptic sphincter being nearly fixed, its orifice can only be obliterated by reducing it to an anteroposterior slit. By means of this movement the two sides of the velum of the palate resemble two curtains drawn together, the pharyngeal muscles, which are concave when in the state of repose, having their curve reduced to a straight line; and representing in this state of contraction the string of the bow which they represented when in the state of repose (Fig. 63; B, 2); an opening still remains, however, more or less wide, but this is obliterated by the contractions of the middle and inferior sphincters of the pharynx. The uvula, finally, is intended to close any crack which may still remain open, but it is not indispensable (Fig. 63, B, 3, 1). By means of these movements, some idea of which was formed by Albinus and Sandifort, though they have been most clearly demonstrated by Gerdy and Dzondi, the occlusion of the isthmus of the fauces is made even hermetical. Indeed, if the nostrils be stopped up during deglutition, we find that hearing is slightly obstructed. This is because, during the succession of peristaltic movements of the pharynx, its upper part is inclined; and, as the pharyngeal sphincter still remains shut, rarefaction of the air in the nasal chambers ensues. But as, during deglutition, the base of the velum of the palate is stretched out and fixed by the contraction of its superior muscles, and thus opens the Eustachian tube, it follows that the rarefaction of the air of the nasal chambers is communicated to the tympanic drum, and kept up until a fresh movement of deglutition places this drum in communication with the freely opened nasal

1 This fact of the rarefaction of the air suggested to Maissiat (1838) a singular theory of deglutition, which has been refuted by the explanation given here. Maissiat maintained that when deglutition takes place a vacuum is formed in the cavity of the pharynx by means of its rising and subsequent enlargement: the food is precipitated into this cavity by the pressure of the atmosphere, forming what Maissiat calls the involuntary jerk (saccade) of deglutition.

This phenomenon does occur; but, in the first place, it is not in the pharynx, properly so called, but in the naso-pharyngeal cavity; and, in the second, the formation of this vacuum does not correspond with the rising of the pharynx, but with its descent; not at the beginning, but at the end, of deglutition.

chambers. This shows how complete is the obliteration of the isthmus of the fauces; and it may also be shown by means of a tube communicating at one end with the nasal chambers (the nostrils being closely pressed against the tube), while the other end is immersed in water (experiment by Maissiat) at each movement of deglutition the water rises in the tube, on account of the rarefaction of the air of the nasal chambers (by the descent of the constricted isthmus of the fauces); this rarefaction is communicated to the air in the tube, as it is to that in the hollow of the tympanum.

The isthmus of the fauces thus undergoes a triple change during deglutition: it closes by the contraction of its constrictors; it rises slightly as deglutition begins; and descends slightly as this is finished. This rising and falling is produced by the simultaneous movement of the pharynx. The descent explains the vacuum produced in the closed nasal chambers: the ascent shows us why a probe introduced horizontally into the nasal chambers, as far back as possible, will be slightly pushed forward as each movement of deglutition. begins (experiment by Debrou); this led Bichat to believe that there was some disarrangement at the top of the velum of the palate, and others that the velum is simply raised; but we have seen that not only the velum, but the whole isthmus of the fauces when constricted, rises and sinks again immediately.

The occlusion of the antero-inferior orifice of communication, or orifice of the larynx, is effected by means of the epiglottis, which, when free, leaves the respiratory orifice uncovered, but, as it is formed of elastic tissue, bends under the weight of the food as it passes. The epiglottis is not, however, indispensable to this obliteration. As the pharynx rises, the larynx, sharing the movement, strikes against the base of the tongue (which is there protuberant), and this mechanism is sufficient to protect the respiratory orifice, or at least to secure the retroversion of the epiglottis over it. The small cartilages placed above the arytenoid cartilages join with the epiglottis in effecting the occlusion of the opening of the larynx.

The absence of the epiglottis is scarcely any hinderance to the deglutition of solids: the movements of the whole larynx under the cushion, at the base of the tongue, suffice to protect the respiratory orifice. The case is not the same, however, with the deglutition of fluids, and this shows us the purpose of the epiglottis. When the deglutition of a mass

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