94 COMPARATIVE PHYSIOLOGY 



greatly with the feeding habits of the organism. In the 

 mammal the bulk of the fat is unchanged in the stomach. 

 In the duodenum, aided by the churning movements of the 

 intestinal wall, bile-salts exert their characteristic effect in 

 lowering surface tension to effect a fine degree of emulsifica- 

 tion. The emulsion is acted upon by a lipolytic enzyme of the 

 pancreatic juice, and broken down completely into its hydro- 

 lysis products by neutralisation of the fatty acid so formed 

 with production of soaps. The soaps of the higher fatty acids 

 form colloidal solutions, and are not diffusible like amino- 

 acids or sugars. They are absorbed by the cells of the mucous 

 membrane, which are richly gorged with fat-globules after a 

 meal of fat- containing food. The colloidal nature of the 

 higher soaps suggests that the same way of dealing with hydro- 

 lysis products of fat should hold in other groups, as the work 

 of Sanford (191 8) on digestion in the cockroaches clearly 

 demonstrates. Sanford fed cockroaches on a mixture of sugar 

 and olive oil and showed, by following microscopically the 

 course of digestion, that after a fatty meal the cells lining the 

 wall of the crop teem with fat globules. The contents of the 

 crop exhibited, like the pancreatic juice of the mammal, a 

 powerful Hpolytic action. This provides good material for 

 class experiment. The organ is removed from about a dozen 

 cockroaches, ground in a mortar with sand and about 10 c.c. 

 of water, a few c.c. of the filtered extract is added to about 

 the same quantity of olive oil and kept for a few days at room 

 temperature, when the amount of free acid liberated is deter- 

 mined by titration and compared with a control tube. For 

 microscopic examination sections of the wall fixed at varying 

 intervals after the meal are treated with osmic acid or the 

 dye known as Sudan III., both of which are specific stains 

 for fat. Sanford also fed cockroaches on a paste of oil and 

 sugar mixed with Nile Blue sulphate, which is absorbed by the 

 fat-globules and gives a red coloration in presence of free 

 fatty acid. On cutting frozen sections a few hours after such 

 a meal, a red mass is seen cHnging to the wall of the crop which 

 is itself blue owing to the dye adsorbed by the fat globules 

 in the cells. 



