THE PART PLAYED by the TONGUE in MASTICATION and DEGLUTITION by SHAFIK ABD-EL-MALEK Department of Anatomy, Abbasia Medical Faculty, Egypt

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THE PART PLAYED BY THE TONGUE IN MASTICATION AND DEGLUTITION BY SHAFIK ABD-EL-MALEK Department of Anatomy, Abbasia Medical Faculty, Egypt

INTRODUCTION Thefunctions ofthe human tongue in tasting, swallowing andspeaking are wellknown. Its assistance in mastication is also well recognized; Gibbons (1898), for example, states that besides controlling, with the assistance of the buccinator the position of the food between the teeth, it also turns the food and mixes it with saliva and aids in separating and sorting out unsuitable particles. As regards the individual movements of the tongue, Harris (1927), from radio- graphic observations using barium milk, sometimes thickened with bread, described movements including pocket formation between the tip of the tongue and the teeth, followed in succession by hollowing of the dorsum, by raising of the anterior half to make contact with the palate and depression of the posterior half, and a final piston- like stroke as the barium was 'shot downwards'. Johnstone (1942) also described the hollowing of the dorsum, but emphasized the forceable and sudden elevation of the hyoid bone and larynx as the tongue was pressed against the palate and the barium projected through the fauces. Whillis (1946) speaks of an anticipatory pro- trusion of the tongue so that its tip comes into contact with the lower lip, followed by retraction carrying the food into the mouth. But it is clear that these complex movements are not yet fully understood, particularly those associated with the distribution of the food during mastication.

METHODS OF INVESTIGATION Observations were made on subjects who had lost some of their teeth, preferably on both sides, one side then being available for illumination and the other for observation. In such subjects the movements of the tongue could be seen by parting the lips with small retractor forceps, so as to expose the teeth on the side under observation. Nuts, coloured gelatin food stuffs and coloured chewing gums were used, and drawings showing positions of the tongue were made for later elaboration. The range of tongue movements was studied photographically with the mouth open. Radio- graphic examination proved less successful, chiefly on account of the speed at which mastication was performed. A few observations on hemiplegic patients were included.

THE SEQUENCE OF MASTICATION AND DEGLUTITION The normal sequence of mastication comprises five stages: (1) The 'preparatory' stage. The tongue being placed in the resting position on the floor of the mouth, with its dorsum looking upwards, prepares itself for the reception of the food by becoming trough-like, so as to collect the foodstuff on its dorsum The part played by the tongue in mastication and deglutition 251 (Text-figs. 1, 2). Pocket formation at the tip and contact with the lip are unusual when solid matter is taken. (2) The 'throwing-stage'. The anterior half, or rather more of the tongue, having collected the foodstuff, twists over on one side, turning through a right angle so that its dorsum is made to face the lingual surface of the teeth (Text-fig. 3). This twisting is carried out by a sudden movement which throws the ingesta on the surface of the lower grinding teeth.

Text-fig. 1. Text-fig. 2. Text-fig. 1. A diagram to show the tongue in the mouth surrounded by the teeth of the lower jaw. It shows the tongue in the first or preparatory stage of mastication or trough-like formation. Text-fig. 2. A diagram to illustrate the tongue assuming a well-formed trough to collect the food before the next throwing stage. (3) The 'guarding' stage. The tongue retains the twisted position and its dorsum presses on the medial side of the grinding teeth to prevent the food slipping medially into the buccal cavity (Text-fig. 4). The tongue and the buccinator muscle of the appropriate side act together to keep the food between the grinding teeth, and prevent its escape, while the teeth cut and grind the ingesta. (4) The 'sorting-out' stage. This begins when, at the completion of a series of chewing movements, the grinding teeth are separated from one another. The buccinator muscle drives the cheek medially so that it bulges between the teeth and pushes the food into the buccal cavity and on to the tongue, which is now preparing to regain its position of rest on the floor of the mouth. The tongue movement is again rapid and jerky, and this may help to sort out the larger particles of food, which still need crushing, on to the middle trough-like part of the tongue, whence they can be replaced between the teeth, while the parts already sufficiently crushed are placed more laterally on the tongue. These movements are repeated as long as any food particles remain uncrushed. (5) The stage of' boluwformation'. This partly overlaps the last. The tongue makes alternate side to side churning movements designed to mix the crushed food with saliva, coat it with mucus, and make it into a bolus ready for swallowing. The tongue now prepares itself for the act of swallowing. 252 Shafik Abd-el-Malek (6) In the first stage of deglutition, that of 'closure', the tip of the tongue is raised and pressed rather hard against the posterior surface of the front teeth and the anterior part of the hard palate, so as to close off the mouth and pharynx. In swallowing fluid the tongue assumes at this stage a deep gutter-like form (Fig. 5).

Text-fig. 3. Text-fig. 4. Text-fig. 3. A diagram to show the tongue twisting to the right, during the second stage of mastication or the 'throwing stage'. Text-fig. 4. A diagram of the tongue in the third or 'guarding stage'; the tongue is pressing on the medial side of both the lower and upper teeth. The ventral surface of the tongue with its twisting frenulum is seen.

Text-fig. 5. A diagram to show the tongue in a gutter-like shape during deglutition of fluids.

(7) Stage of 'slide preparation'. The hyoid bone is pulled sharply upwards and anteriorly, and the posterior part of the tongue is depressed towards the hyoid bone. The tongue now assumes a special position and form. It slopes obliquely downwards and backwards, its tip is pressed against the front teeth and hard palate, and its root against the hyoid bone. It is rendered concave both from side to side, The part played by the tongue in mastication and deglutition 253 especially near the tip, and also antero-posteriorly, so as to present a smooth slope leading from the mouth to the oropharynx. (8) Stage of 'pressure'. The bolus is forced by pressure between the tongue and palate to move backwards towards the isthmus and over the posterior part of the tongue as it enters the pharynx.

THE MUSCLES CONCERNED Depression of the tongue near the mid-line and raising of its lateral margins are produced by contraction of the genio- and hyoglossi combined with bilateral action of the styloglossi (P1. 1, fig. 1, in which the tongue is also protruded to bring it within view of the camera). When the tip is turned sharply upwards the superior longitudinal muscles come to stand out as a pair of thick parallel bands lying mid-way between the median plane and the lateral margins of the tongue (PI. 1, fig. 2), and a weaker action of these muscles is involved in the formation of the trough-like 'preparatory' stage. When in addition the tongue is elongated a groove forms along its lateral margins (PI. 1, fig. 3), due to the strong action of the transverse musculature, which, by reducing the transverse diameter at a time when the vertical diameter is already restricted by the genio- and hyoglossi, forces the tongue to elongate in the longitudinal direction. The twisting of the tongue found in the 'throwing' and 'guarding' stages is brought about chiefly by the unilateral action of the styloglossus, the left muscle throwing the ingesta between the right grinding teeth. Twisting the tongue brings the margin under which the appropriate muscles lie into strong prominence (PI. 1, fig. 4). The tip of the tongue can be twisted independently of the main body (PI. 1, fig. 5), probably by unilateral action of the inferior longitudinal muscle, but this cannot be determined by palpation. The pressure of the tip of the tongue on the palate in the early stages of deglutition may be assisted by the lingual cap, a thickening of the lamina propria of the mucous membrane near the tip of the tongue (Abd-el-Malek, 1939). Contraction of the intrinsic muscles of the tongue within the cap causes it to stand out on the surface and become sharply defined from the remainder of the tongue, particularly on the dorsum (PI. 1, fig. 6). When the tip is made firm in this manner it forms a rela- tively rigid mass that can be pressed against the teeth or palate by other muscles. The cap is maintained in its firmed condition as the ingesta are forced to move by pressure. The longitudinal muscles working from the firmed cap as an origin may help to pull the back of the tongue anteriorly in the stage of 'slide preparation'.

CLINICAL OBSERVATIONS In patients suffering from tongue injuries such as severe wounds or painful scars, or after operations on the tongue involving the removal of half its substance, a stomach tube is essential for feeding. In normal conditions it is equally easy to masticate on either side, but in cases of hemiplegia of the right side, the patient cannot, at the beginning of his illness, masticate on the left unparalysed side though the mandibular muscles on that side are still healthy and active. This is because he cannot throw the ingesta between the left grinding teeth, owing to the impaired 254 Shafik Abd-el-Malek function of the muscles of the right half of the tongue. This explains why these patients, suffering from right hemiplegia, cannot avoid accumulation of food on the right side of the vestibule of the mouth. They feel discomfort as they are unable to masticate the food on the healthy side unless they place it between the teeth by hand, and even then after the first crushing movement it spreads between the right teeth. The bite on the paralysed side is weak as it has to be carried out by the muscles of mastication of the opposite side. In cases of epithelioma of the tongue, if the jaws are left intact, the patient can, after recovery from operation, only masticate on the affected side, for it is only to this side that his tongue can throw the food, though he can bite on the healthy side normally. Swallowing also may be difficult, the bolus escaping to the paralysed side where it may be retained in the pyriform area.

SUMMARY 1. The action of the tongue in mastication has been observed in subjects who have lost some of their teeth. 2. The tongue takes part in all stages, defined as these of 'preparation' when the tongue becomes trough-like, 'throwing' the food between the teeth, 'guarding' the food from falling back from the teeth, 'sorting out' the particles and 'bolus formation', and in swallowing. 3. The muscles of the right side of the tongue throw the food between the left grinding teeth and prevent it from escaping during chewing movements, and this explains why, in hemiplegia the patient is forced to chew on the paralysed side.

I wish to express my thanks to Prof. R. W. Haines and Prof. H. A. Harris for their help and criticism. REFERENCES ABD-EL-MALEK, S. (1939). Observations on the morphology of the human tongue. J. Anat., Lond., 73, 201-210. GIBBONS, J. W. (1898). The human tongue in mastication and swallowing. Brit. J. dent. Sci. 41, 869-879. HARRIS, P. (1927). X-ray study of movements of the tongue. Laryngoscope, St Louis, 37, 235-239. JOHNSTONE, A. S. (1942). A radiological study of deglutition. J. Anat., Lond., 77, 97. WHILLIS, J. (1946). Movements of the tongue in swallowing. J. Anat., Lond., 80, 115-116.

EXPLANATION OF PLATE 1 Fig. 1. The tongue with depressed centre and raised margins, as in preparation for the reception of food. Fig. 2. Longitudinal ridges due to strong action of the superior longitudinal muscles. Fig. 3. Lateral grooves due to the action of the transverse musculature. Fig. 4. Twisting of the tongue by the unilateral action of the styloglossus. Fig. 5. Independent twisting and down-turning of the tip of the tongue. Fig. 6. The lingual cap, with its concave posterior margin, is outlined on the dorsum of the tongue. Journal of Anatomy, Vol. 89, Part 2 Plate I

AHD EL-IMALEK THE PART PLAYED BY THE TONGUE IN MASTICATION AND DEGLUTITION (Facing p. 254)