DEGLUTITION. 313 



evident that deglutition must be a much more complex action. In that 

 position the mylo- and genio-hyoid muscles are relaxed, and cannot act 

 efficiently in drawing the hyoid bone upward and forward so as to 

 allow the masticated mass to pass into the oesophagus, into which it has 

 to pass, in fact, around an angle. The difficulty is removed by the con- 

 nection of the digastric muscle with the hyoid bone. This muscle, 

 during the act of deglutition, causes the hyoid bone, larynx, and base of 

 the tongue to move through a segment of a circle, the anterior part of 

 the muscle drawing these parts forward ; they are then elevated by the 

 joint action of the anterior and posterior bellies, and finally drawn back- 

 ward by the posterior bellies, so as to force the masticated mass into 

 the oesophagus. 



The second stage of deglutition is facilitated by the mucous secre- 

 tions of the parts concerned. This secretion may become enormous, as 

 in the dromedary, where the appendix to the soft palate and the pharyn- 

 geal pouch are very glandular. In all animals the secretion of the 

 mucous membrane of the mouth and pharynx, aided by the salivary 

 secretion, is amply sufficient to lubricate the food, so as to render deglu- 

 tition possible. It was already noted in the chapter on the salivary 

 secretion that the quantity of saliva poured out was largely dependent 

 upon the character of the food ; or, in other words, the drier the food 

 the greater the amount of lubricant needed, and, therefore, the greater 

 was the salivary secretion. 



The second stage of deglutition is involuntary, and when the bolus 

 of food has passed beyond the anterior pillars of the fauces it is no 

 longer within the control of the will, and can only be returned to the 

 mouth by vomiting or violent coughing. Therefore, in giving pills or 

 balls to animals they have to be carried mechanically by the hand behind 

 the pillars of the fauces ; they are then carried down to the stomach by 

 the involuntary contraction of the pharynx and oesophagus. 



The third stage of deglutition occurs after the food has passed 

 through the pharynx and has entered into the oesophagus. This stage 

 of deglutition is much more prolonged than the two preceding stages, 

 and in the larger domestic animals the passage of the food by the 

 oesophagus may be followed by the eye and touch. Where the secretion 

 of saliva is scanty the duration of this stage becomes prolonged, and 

 sometimes, as in the horse, the food may become arrested in the lower 

 cervical portion of the oesophagus until pushed on by the next succeed- 

 ing bolus. 



The rapidity of motion in the oesophagus varies. Liquids and very 

 soft foods are very rapidly swallowed, being actually squirted through 

 the oesophagus ; dry forage is swallowed very slowly. In the horse the 

 boluses have to be very small, from the narrow character of the gullet 



