148 THE BIOLOGY OF THE FROG chap. 



less respectively in the pieces from the other regions named. 

 Partsch, Nussbaum, Swiecicki, Langley, and Sewall have all 

 investigated the relative digestive power of the glands well 

 filled with granules and glands from which the granules have 

 mainly disappeared, and all agree that the pepsin content of 

 the former is much the greater. 



Rapidity of Digestion. — The digestive processes of the 

 frog compared with those of the higher vertebrates proceed 

 slowly, due probably to the fact that the frog is a cold-blooded 

 animal. The length of time taken to digest a meal varies 

 with the amount of food. Langley found that a small earth- 

 worm was digested in somewhat less than twenty-four hours, 

 but if several worms were given, they do not disappear from 

 the stomach until a longer period. It is very probable that 

 the temperature of the body is an important factor in deter- 

 mining the rate of digestion, but I am acquainted with no 

 observations to that effect. 



Structure of the Intestine. — The small intestine begins 

 just behind the pyloric constriction, and runs forward as the 

 duodenum for some distance, when it turns abruptly backward 

 as the ileum, which after coiling about in an irregular manner, 

 widens out abruptly into the large intestine near the posterior 

 end of the body. The diameter of the small intestine, which 

 is nearly uniform throughout its course, is much less than that 

 of the stomach, and its walls are much thinner. The intestine 

 is fastened by a mesentery to the mid-dorsal portion of the 

 body cavity, and its duodenal portion is connected to the 

 liver and stomach by the previously mentioned remains of a 

 ventral mesentery, the gastro-hepato-duodenal ligament. 



A cross section of the small intestine shows the following 

 layers : At the outside is a very thin coat of peritoiieum 

 similar to that coating the stomach. Within this is a well- 

 marked layer of longitudinal muscle fibers ; then comes a 



