OF DIGESTION. 153 



operate in various ways to aid it are the assistant digestive appara- 

 tus. Besides, we have a set of organs for preparing the food for 

 the stomach, tearing, bruising, grinding and mixing it with fluid 

 that it may be easily swallowed. It is most convenient, though 

 perhaps not most philosophical, to trace these parts successively 

 from the mouth downward ; we will, therefore, commence with the 

 parts concerned in digestion, inquire into the nature of the action 

 of swallowing, and trace the food as far as the stomach. From the 

 stomach we will follow it down into the small intestines, see how 

 it is mingled with the bile and the pancreatic juice, and learn how 

 the nourishing parts of it are absorbed and carried into the circu- 

 lating mass, and how that part which is useless is pushed on, until 

 it is expelled from the body. 



MOUTH. 



The mouth is a cavity having somewhat the shape of a hemi- 

 sphere, the flat surface being directed downward and the con- 

 vexity upward. Its roof is formed by bone ; it is shut in at the 

 sides and front by the muscular parts forming the cheeks and lips ; 

 below, it is enclosed round about by the lower jaw, and its floor is 

 formed by the tongue. From the back part of the mouth hangs 

 down a conical bag, which leads into the oesophagus or gullet, and 

 the nose communicates with it from above. Hence is the reason 

 that we can breathe equally by the nose or mouth, and that some- 

 times if we are taken by surprise with a fit of coughing while swal- 

 lowing, the contents of the throat run out through the nose. In 

 order to- prevent this from occurring constantly, there is a curtain 

 placed at the back of the mouth, which we see on looking into a 

 glass, and which rises or falls according to the necessity for its 

 being applied either above or below. A long red tassel hangs 

 down from the centre of it, nearly touching the top of the tongue, 

 which is endowed with great sensibility, and warns the curtain to 

 rise whenever the food comes in contact with it. When food is 

 about to be swallowed, it is rolled about in the mouth and mixed 

 with saliva, till it forms a kind of ball, and when it gets to the back 

 of the mouth, between the arches of the palate, there is felt an irre- 

 sistible tendency to swallow. The curtain now rises so as to pre- 

 vent any of it passing up into the nose, the tongue rises against the 

 roof of the mouth, so as to keep it from getting, forward again, and 

 the only course left for it is to pass down into the gullet. It is a 

 mistake, however, to suppose that food falls into the stomach, tho 



