STRUCTURE OF THE STOMACH. 265 



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 pos- 

 terior 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 

 oesophagus, 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 pneumogastric 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 an internal cavity for the production of a 

 chemical change in the aliment a cavity for digestion ; 

 and when this cavity is compound, the 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 intermediate muscular coat, with blood-vessels, 

 lymphatics, and nerves distributed in and between them. 



The muscular coat of the stomach consists of three sepa- 

 rate layers or sets of fibres, which, according to their several 

 directions, are named the longitudinal, circular, and oblique. 

 The longitudinal set are the most superficial : they are con- 



