CHAPTER XXXVII 



THE MECHANICAL PROCESSES OF DIGESTION 



UNDER this head we shall study the neuro-muscular mechanism of the 

 alimentary canal, which has for its object the onward movement of 

 the food, and its thorough admixture with the digestive juices. We 

 shall therefore have to consider mastication, deglutition, the move- 

 ments of the stomach and intestines, defsecation, and vomiting. 



MASTICATION. 



The act of mastication is performed by the biting and grinding 

 movement of the lower range of teeth against the upper. The 

 simultaneous movements of the tongue and cheeks assist partly by 

 crushing the softer portions of the food against the hard palate and 

 gums, and thus supplement the action of the teeth, and partly by 

 returning the morsels of food to the teeth again and again, as 

 they are squeezed out from between them, until they have been 

 sufficiently chewed. 



The act of mastication is much assisted by the saliva, and the 

 intimate incorporation of this secretion with the food is called 

 insalivation. 



Mastication is much more thoroughly performed by some animals 

 than by others. Thus, dogs hardly chew their food at all, but the 

 oesophagus is protected from abrasion by a thick coating of very 

 viscid saliva which lubricates the pieces of rough food. 



In vegetable feeders, on the other hand, insalivation is a much 

 more important process. This is especially so in the ruminants ; in 

 these animals, the grass, etc., taken, is hurriedly swallowed, and passes 

 into the first compartment of their four-chambered stomach. Later 

 on, it is returned to the mouth in small instalments for thorough 

 mastication and insalivation ; this is the act of rumination, or 

 " chewing the cud " ; the food is then once more swallowed, and 

 passes on to the digestive regions of the stomach. 



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