STRUCTURE OF THE STOMACH. 273 



the passage into the tipper part of the pharynx and the 

 posterior nares, and form an incline 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 progres- 

 sive contraction of the oesophagus is slow, and attended 

 with pain. Division of both pneumogastric nerves para- 

 lyzes the contractile power of the oesophagus, and food 

 accordingly accumulates in the tube (Bernard). 



. DIGESTION OP FOOD IN THE STOMACH. 



Structure of the Stomach. 



It appears to be an almost universal character of ani- 

 mals, that they have an internal cavity for the production 

 of a chemical change in the aliment a cavity for diges- 

 tion ; 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, lym- 

 phatics, 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 continuous with the longitudinal fibres of the 



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