Digestion. 99 



It is a remakable fact that in all herbivora the teeth 

 never leave off growing ; if one molar be removed its fellow 

 in the lower jaw, through the absence of friction, grows 

 into the space left. This is a practical fact of clinical im- 

 portance. In the incisor teeth the changes due to constant 

 growth assist in determining the age. 



Deglutition comprises three stages. In the first stage 

 the food is passed backwards by means of the tongue 

 under the soft palate, which is raised to allow it to pass ; 

 in the second stage, the tongue is drawn upwards and 

 backwards, the pharynx and larynx are advanced, and the 

 base of the tongue and bolus close the epiglottis. The soft 

 palate, which in the horse reaches well into the pharynx, is 

 still up and cuts off the communication with the nose ; 

 without this provision both food and water would return by 

 the nostrils. In the third stage, the bolus is grasped by the 

 pharyngeal muscles, which contract on it from before back- 

 wards and gradually pass it along the u'sophagus, either 

 with or against gravity. In the horse and ox the passage 

 of the bolus along the channel of the neck can be dis- 

 tinctly seen. 



Part of the act is under the control of the will, but once 

 the food reaches the fauces the action is entirely reflex ; the 

 closure of the glottal opening and the presence of food in 

 the pharynx excites the action of swallowing, which is a 

 purely reflex condition the centre for which exists in the 

 medulla. The pharyngeal muscles are remarkably strong 

 and stoutly grip the bolus. 



Though the epiglottis closes the glottal opening, this 

 closure is not absolutely essential to swallowing. When the 

 epiglottis has been removed the base of the tongue takes 

 its place ; but horses can swallow when the glottis is open 

 — for example, when the arytenoid cartilage has been re- 

 moved. 



Owing to the length of the soft palate in the horse, food 

 rejected from the stomach or unable to pass along the 

 (esophagus can only be returned by the nostrils. 



Though the passage cf food along the oesophagus is com- 



7—2 



