DIGESTION. 



133 



materials it contains. The stomacli is, in most animals, a 

 simple sac, composed of several membranes, enclosing thin 

 layers of muscular fibres, abundantly supplied with blood- 

 vessels and with nerves, and occasionally containing struc- 

 tures which appear to be glandular. The human stomach, 

 which is delineated in Fig. 301, exhibits one of the simplest 



form^of this organ; c being the cardiac portion, or part 

 where the oesophagus opens into it; and p the pyloric por- 

 tion, or that which is near its termination in the intestine. 

 At the pylorus itself, the diameter of the passage is much 

 constricted, by a fold of the inner membrane, which is sur- 

 rounded by a circular band of muscular fibres, performing 

 the office of a sphincter, and completely closing the lower 

 orifice of the stomach, during the digestion of its contents. 

 The principal agent in digestion, as far as the ordinary 

 chemical means arc concerned in that operation, is a fluid 

 secreted by the coats of the stomach, termed the Gastric 

 juice. This fiuid has, in each animal, the remarkable pro- 

 perty of dissolving, or, at least, reducing. to a pulp, all the 

 substances which constitute the natural food of that particu- 

 lar species of animal; w'hile it has comparatively but little 

 solvent power over other kinds of food. Such is the con- 

 clusion which has been deduced from the extensive re- 

 searches on this subject, made by that indefatigable experi- 

 mentalist, Spallanzani, who found, in numberless trials, that 

 the gastric juice taken from the stomach, and put into glass 



