182 THE VITAL FUNCTIONS. 



a simple sac, composed of several membranes, 

 enclosing thin layers of muscular fibres, abun- 

 dantly supplied with blood-vessels and with 

 nerves, and occasionally containing structures 

 which appear to be glandular. The human sto- 

 mach, which is delineated in Fig. 301, exhibits 



one of the simplest forms of this organ ; c being 

 the cardiac portion, or part where the oesopliagus 

 opens into it ; and p the pyloric portion, or that 

 which is near its termination in the intestine. 

 At the pylorus itself, the diameter of the pas- 

 sage is much constricted, by a fold of the inner 

 membrane, which is surrounded 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 are concerned in that 

 operation, is a fluid secreted by the coats of the 



