DIGESTION 137 



blood-vessel*. We have seen how the inner membrane of the 

 stomach is underlaid by a tracery of minute and numerous 

 vessels, and how some portions of the food are by them 

 absorbed. The supply of blood-vessels to the intestines is 

 even greater; particularly to the small intestines, .where the 

 work of absorption is most actively carried on. 



41. The absorbing surface of the small intestines, if con- 

 sidered as a plane surface, amounts to not less than half a 

 square yard. Besides, tSe mucous membrane is formed in 

 folds with an immense number of thread-like prolongations, 

 called v #//,' which indefinitely multiply its absorbing capacity. 

 These villi give the surface the appearance and smoothness of 

 velvet, and during digestion they dip into the canal, and, by 



8. Experiment. Pancreatic Juice. To obtain the pancreatic juice, 

 it will be necessary to extract it from a fresh pancreas, " sweet-bread " 

 (which may be bought of any butcher), by means of either water or 

 glycerin. -Mince the pancreas finely and add sufficient of either of these 

 liquids to cover it, set aside for a day and strain or filter. The glycerin 

 extract will keep for a long time ; the watery extract soon spoils. 



The pancreatic juice has a -digestive action on all the food-stuffs. 



Its action upon boiled starch may be demonstrated by repeating the 

 experiments detailed under " Saliva," only substituting the extract of the 

 pancreas for the saliva. 



Its action upon albuminoids is shown by adding to a few teaspoonfuls 

 of the pancreatic extract some finely minced meat, or a part of the 

 white of a boiled egg. But as the action of the pancreatic juice can 

 only proceed in an alkaline medium, it is necessary to be certain of the 

 alkalinity of the fluid. This is be.st done by adding about five grains of 

 carbonate of soda (washing soda) to each ounce of pancreatic extract. 

 Set the test-tube aside in a warm place for a few hours, when the meat 

 or white of egg will be found entirely dissolved. 



Its action upon fats may be shown by shaking up in a test-tube a 

 quantity of the extract with an equal part of melted lard or oil, when 

 we will obtain a very perfect emulsion. 



40. What completes the work ofdisrestion ? How much thus far has been done with the 

 food? What remains to be done* What is this process called? In what two ways is it 

 accomplished ? By what is the inner membrane of the stomach underlaid ? What purpose 

 does it serve? Are the intestines supplied with corresponding blood-vessels? In greater 

 or less quantity ? Where is the work of absorption most actively carried on ? 



41. Considered as a plane surface, what is the. extent of the absorbing surface of the 

 small intestines? Describe the formation of the mucous membrane. What are the villi ? 

 In what way do the, villi affect the surface of the membrane? What is their office during 

 digestion ? 



