312 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Coats. — The stomach consists of three coats — an 

 investing serous coat, very smooth, thin, and tough ; a 

 middle muscular coat ; and a lining mucous coat. The mus- 

 cular coat is by far the thickest, so that the organ may 

 be called a hollow muscle, invested with serous mem- 

 brane and lined with mucous membrane. The function 

 of the serous coat is to give toughness and easy gliding 

 without friction over other organs in the abdomen. The 

 function of the muscular coat is to do the mechanical 'work, 

 and of the mucous coat the chemical work of digestion. 



Mechanical Work ; Chymification. — The me- 

 chanical work of mixing the food with the gastric juice 

 is done by the muscular coat. This consists of several 

 sheets of parallel fibers running in different directions, 

 longitudinal, transverse or ring, and oblique fibers. 

 Under the contraction of these the stomach is seen to 

 squirm, and especially to transmit light waves of annu- 

 lar contraction, chasing one another from the cardiac to 

 the pyloric end. By these contractions the food is gen- 

 tly urged along the greater curvature to the pylorus and 

 back along the lesser curvature to the cardiac orifice, 

 and so on repeatedly until the digestion is complete. 



The pylorus (gate keeper) is a strong collection of 

 circular fibers at the outgoing orifice of the stomach. 

 During digestion it is completely closed. After two to 

 five hours, depending on the nature of the food and the 

 digestive power of the stomach, the digested food is 

 allowed to pass the pylorus; but if any undigested por- 

 tions appear the pylorus closes, and sends it on its way 

 round again until in a proper condition. But sometimes, 

 by repeated application, the gate keeper is, as it were, 

 teased into compliance, and even undigested matter may be 

 allowed to pass, and may create trouble farther on. 

 The final result of this process is a grayish, slightly acid, 

 semiliquid mass, about the consistence and somewhat 



