28 PHYSIOLOGY OF FARM ANIMALS [CH. 



the dog in being able to move the jaw laterally as well as up and 

 down. Thus the articulation of the lower jaw in these animals 

 is provided with a well-developed cartilaginous disc which is 

 sufficiently yielding to admit of movement in different directions. 



Mastication is very thoroughly performed in the horse, which 

 may take as much as ten minutes to eat a jDound of corn, or 

 twenty minutes to eat a pound of hay. In ruminating animals 

 mastication is performed principally after the cud has been re- 

 turned to the mouth. In the dog the food is only very slightly 

 masticated. 



In the horse and ox and other herbivorous animals mastica- 

 tion only takes place on one side at a time. To facilitate this 

 unilateral mastication the upper jaAv is wider than the lower; 

 otherwise the molar teeth would not properly meet during the 

 process. A horse or an ox will masticate on one side only for 

 many minutes at a time before transferring the food to the 

 molars on the opposite side. 



In drinking, the tongue is drawn backwards, while the lips 

 are shut excepting for a small orifice in front which is placed 

 under the surface of the water. The water is thus sucked in by 

 the action of the tongue, the cheeks being drawn inwards at the 

 same time. The act of sucking as performed by young animals 

 is essentially similar, the size of the tongue being decreased in 

 front and increased behind. A horse while drinking extends its 

 head, moving its ears forwards during each swallow and letting 

 them fall back in the intervals between the swallows. Lapping 

 as performed by the dog and cat is carried out by the animal 

 curling its tongue and using it like a spoon and so conveying the 

 liquid into the mouth, the tongue being withdrawn and extended 

 in a succession of rapid movements. 



Deglutition or the act of swallowing occurs normally in three 

 stages. In the first stage the food is carried back to the base of 

 the tongue by the action of the tongue muscles. In the second 

 stage the larynx is pulled upwards and its entrance closed by 

 the pressure of the tongue on the cartilaginous epiglottis which 

 guards the entrance to the windpipe. The posterior nares which 

 communicate with the nostrils also become closed In' muscular 

 action, and the food is grasjoed by the muscles of the j^har^mx 

 and forced into the oesophagus. In the third stage of swallow- 

 ing the food is carried doAvn the oesophagus by successive waves 



