ORGANS OF DIGESTION I47 



uterus pass from the sides of the uterus to the brim of the 

 pelvis. 



The peritoneum is called a serous membrane because of the 

 colorless serum secreted by it. Other serous membranes are 

 the pericardium of the heart, the pleura lining the thoracic 

 cavity and investing the lungs, and the synovial membranes 

 of the joints. Serous membranes invest only such cavities as 

 are not exposed to the air. All other cavities are lined by 

 mucous membrane. 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 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 



Tdir 



Fig. 73. — Diagram of the Stomach of a Ruminant. 

 a. Dotted line showing the direction of the food in the process of digestion; 

 abom, abomasum; du, duodenum; oe, esophagus; ps, psalterium; rt, reticulum. 



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, 

 psalterium or many plies, and ahomasum (Fig. 73). The rumen 

 and reticulum serve as mere storage cavities, from which 

 the food returns to the mouth for thorough mastication, and 

 then passes direct, by means of a groove in the esophagus, into 

 the psalterium, and finally on to the abomasum. The latter 

 is the true stomach, and is provided with gastric glands. In 



