162 THE VITAL FUNCTIONS. 



The preparatory changes we have lately been 

 occupied in considering, consist chiefly in the reduc- 

 tion of the food to a soft consistence, which is accom- 

 plished by destroying the cohesion of its parts, and 

 mixing them uniformly with the fluid secretions of 

 the mouth ; effects which may be considered as 

 wholly of a mechanical nature. The first real 

 changes in its chemical state are produced in the 

 stomach, where it is converted into a substance 

 termed Chyme; and the process by which this first 

 step in the assimilation of the food is produced, 

 constitutes what is properly termed Digestion. 



Nothing has been discovered in the anatomical 

 structure of the stomach tending to throw any light 

 on the means by which this remarkable chemical 

 change is induced on the materials it contains. 

 The stomach is in most animals a simple sac, com- 

 posed of several membranes, enclosing thin layers 

 of muscular fibres, abundantly supplied M'ith blood- 

 vessels and with nerves, and occasionally containing 

 structures which appear to be glandular. The 

 human stomach, which is delineated in Fig. 301, 



' 301 



exhibits one of the simplest forms of this organ ; c 

 being the cardiac portion, or part where the oeso- 

 phagus opens into it ; and p the pyloric portion^ or 



