THE DIGESTIVE SYSTEM 85 



is made these glands are not readily seen, as they are embedded 

 in the intestinal walls. The use of their secretions will be men- 

 tioned when digestion is studied. 



MOVEMENTS OF THE DIGESTIVE ORGANS 



It is evident that movements must occur in the alimentary canal 

 to convey the food from organ to organ and mix it with the 

 various digestive juices. With the exception of the movements 

 in the mouth, pharynx, and anterior end of the esophagus, all 

 are carried on by involuntary muscles. 



Prehension is the act of taking food and drink into the mouth. 

 In manger feeding the horse uses the lips, but the incisor teeth 

 are brought into play to nip off the grass in grazing. The tongue 

 aids in the prehension of fluids either as a piston during sucking, 

 or as a ladle during lapping. Cattle employ the tongue in 

 grazing; sheep use their lips for this purpose. 



Mastication is a voluntary act for the purpose of reducing 

 the larger particles of food to a size suitable to be swallowed 

 and of mixing the food with saliva. The lips, cheeks, and tongue 

 keep the food between the teeth, while the strong muscles that 

 move the jaws are the active agents of mastication. In the 

 horse the movements of the jaws are from side to side and the 

 food is ground between the molar teeth. It is possible for 

 horses to masticate on but one side at a time on account of the 

 upper jaw being wider than the lower. The horse chews his 

 food leisurely, each mouthful being retained for sometime before 

 it is swallowed. Under ordinary conditions it takes him about 

 1)4, hours to eat 6 pounds of dry hay and more than \^ hour to 

 eat the same weight of oats. In the ox the masticatory move- 

 ments are more complex, the mandible moving transversely, 

 longitudinally, and vertically, besides being rotated to some 

 extent. In the llama the lateral movements are regularly alter- 

 nated from one side to the other with each contraction of the 

 principal muscles of mastication. 



Deglutition, or the act of swallowing, is a complicated reflex 

 movement that is mostly performed independently of the will 

 under a stimulus received from the swallowing center situated in 

 the brain. After the food has been masticated it is rolled by the 

 tongue into a sort of ball or bolus. Direct observations by 

 Sisson show that when the horse swallows the following move- 



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