312 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



as the deer, is very slight in the horse and most ruminants, and has more 

 of a forward motion, serving simply to bring the larynx beneath the 

 base of the tongue. The elevation of the larynx serves partly to prevent 

 the passage of food into the larynx. As the larynx is elevated the 

 arytenoid cartilages and both true and false vocal cords are approximated, 

 and as the thyroid cartilage ascends by the action of the laryngeal 

 muscles the epiglottis is depressed to cover the glottis. In this latter 

 operation the depression of the epiglottis is a passive and not an active 

 movement, the depression being due not to an action of any intrinsic 

 muscles, but to the ascent of the larynx beneath the epiglottis, and the 

 mechanical pressing downward of the epiglottis by the weight of the 

 bolus of food and the descent of the root of the tongue. The epiglottis 

 is not, however, essential to the prevention of the entrance of food 

 into the air-passages, since excision of the epiglottis in the dog, or its 

 removal by disease in man, does not interfere with normal movements 

 of deglutition. Colin has found, by inserting a finger into the larynx 

 through an opening in the trachea of a horse which was swallowing, that 

 the larynx at the moment of swallowing was suddenly elevated and 

 moved anteriorly toward the base ef the tongue, the vocal cords closed, 

 and the arytenoid cartilages came in contact with each other. Ity these 

 means food is prevented entering the larynx. He also found, by making 

 a fistula in the upper part of the oesophagus of an ox and inserting a 

 finger, that at each movement of swallowing the epiglottis was depressed, 

 and the entrance to the oesophagus elevated and thus approximated to 

 the isthmus of the fauces. In the horse the isthmus of the fauces is 

 very narrow, and the bolus passes with difficult} 7 , even if not very large, 

 and is often arrested behind the larynx, and yet does not cause coughing. 

 This is often seen after giving a bolus to a horse, particularly in cases of 

 angina. In ruminants the isthmus of the fauces is large and the phaiynx 

 is ample, and when the food sticks in the throat in these animals it is 

 usually in the cervical or thoracic portions of the gullet, which is also 

 the locality where the food is apt to be arrested in the pig. 



In those animals which habitually swallow their food while the head 

 is bent forward, the digastric, in addition to its functions in depressing 

 the lower jaw, is also an aid to deglutition. Where, as in reptiles, birds, 

 and most mammals, the. position of the mouth with respect to the 

 oesophagus during the act of swallowing the food is almost in the same 

 right line, deglutition is easily effected by the mylo- and genio-hyoid 

 muscles drawing the hyoid bone and larynx forward and upward so as 

 to allow the masticated mass to get behind them, and so bring it within 

 the grasp of the pharyngeal muscles ; but in those animals which feed 

 while in the erect or semi-erect position, and the head bent forward so 

 that the cavity of the mouth is at right angles with the oesophagus, it is 



