214 DIGESTION. 



cause the food must pass by the posterior orifice of the nose 

 and the upper opening of the larynx without touching them. 

 When it has been brought, by the first act, between the an- 

 terior arches of the palate, it is moved onwards by the tongue 

 being carried backwards, and by the muscles of the anterior 

 archescontracting on it and then behind it. The root of the 

 tongue being retracted, and the larynx being raised with the 

 pharynx and carried forwards under the tongue, the epiglottis 

 is pressed over the upper opening of the larynx, and the mor- 

 sel glides past it ; the closure of the glottis being additionally 

 secured by the simultaneous contraction of its own muscles : so 

 that, even when the epiglottis is destroyed, there is little danger 

 of food or drink passing into the larynx so long as its muscles 

 can act freely. At the same time the raising of the soft palate, 

 so that its posterior edge touches the back part of the pharynx, 

 and the approximation of the sides of the posterior palatine 

 arch, which move quickly inwards like side curtains, close the 

 passage into the upper part of the pharynx and the posterior 

 nares, and form an inclined plane, along the under surface of 

 which the morsel descends; then the pharynx, raised up to 

 receive it, in its turn contracts, and forces it onwards into the 

 oesophagus. 



In the third act, in which the food passes through the oesoph- 

 agus, every part of that tube as it receives the morsel and 

 is dilated by it, is stimulated to contract : hence an undulatory 

 contraction of the oesophagus, which is easily observable in 

 horses while drinking, proceeds rapidly along the tube. It is 

 only when the morsels swallowed are large, or taken too quickly 

 in succession, that the progressive contraction of the oesophagus 

 is slow, and attended with pain. Division of both pneumo- 

 gastric nerves paralyzes the contractile power of the oesophagus, 

 and food accordingly accumulates in the tube (Bernard). 



DIGESTION OF FOOD IN THE STOMACH. 



Structure of the Stomach. 



It appears to be an almost universal character of animals, 

 that they have ^n internal cavity for the production of a 

 chemical* change in the aliment a cavity for digestion ; and 

 when this cayity is compound, th'e part in which the food 

 undergoes its principal and most important changes is the 

 stomach. 



In man and those Mammalia which are provided with a 

 single stomach, its walls consist of three distinct layers or coats, 

 viz., an external peritoneal, an internal mucous, and an inter- 



