ORGANS OF DIGESTION. 



137 



the synovial membranes of the joints. Serous mem- 

 branes invest only such cavities as are not exposed to 

 the air. All other cavities are lined by mucous mem- 

 brane. A serous membrane consists of a layer of fibrous 

 connective tissue covered by plate-like endothelial cells. 



REMARKS ON THE MAMMALIAN DIGESTIVE SYSTEM. 



The digestive system of mammals varies somewhat 



among the different groups. In carnivorous animals, as 



a rule, the alimentary canal is much shorter than in 



FIG. 66. DIAGRAM OF THE STOMACH OF A RUMINANT. 



a, Dotted line showing the direction of the food in the process of diges- 

 tion; abom, abomasutn ; tin, duodenum; ce, esophagus; ps, psalterium ; 

 rt, reticulum. 



herbivorous. In the wildcat the canal is four times the 

 length of the body, while the canal of the sheep is twenty 

 times as long as the body. In man it is about nine 

 times as long as the distance from the crown of the 

 head to the coccyx. In Ruminants, such as the cow and 

 the sheep, the stomach is very large and divided into 

 four chambers the rumen or paunch, reticulum, psal- 

 terium or many plies, and abomasum. (Fig. 66). The 

 rumen and reticulum serve as mere storage cavities, 

 from which the food returns to the mouth for thorough 



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