300 COMPARATIVE PHYSIOLOGY. 



greater than can be accounted for by the presence of the acid it 

 contains merely, and it has a marked antiseptic action. 



Digestive processes may be conducted out of the body in a 

 very simple manner, which the student may carry out for him- 

 self. To illustrate by the case of gastric digestion : The mucous 

 membrajie is to be removed from a pig's stomach after its sur- 

 face has been washed clean, but not too thoroughly, chopped 

 up fine, and divided into two parts. On one half pour water 

 that shall contain '2 per cent hydrochloric acid (made by add- 

 ing 4 to 6 cc. commercial acid to 1,000 cc. water). This will 

 extract the pepsin, and may' be used as the menstruum in which 

 the substance to be digested is placed. The best is fresh fibrin 

 whipped from, blood recently shed. 



Since the fluid thus prepared will contain traces of peptone 

 from the digestion of the mucous membrane, it is in some 

 respects better to use a glycerin extract of the same. This is 

 made by adding some of the best glycerin to the chopped up 

 mucous membrane of the stomach of a pig, etc., well dried with 

 bibulous paper, letting the whole stand for eight to ten days, 

 filtering through cotton, and then through coarse filter-paper. 

 It will be nearly colorless, clear, and powerful, a few drops 

 sufficing for the work of digesting a little fibrin when added to 

 some two per cent of hydrochloric acid. 



Digestion goes on best at about 40° C, but will proceed in 

 the cold if the tube in which the materials have been placed is 

 frequently shaken. It is best to place the test-tube containing 

 them in a beaker of water kept at about blood-heat. Soon the 

 fibrin begins to swell and also to melt away. . 



After fifteen to twenty minutes, if a little of the fluid in the 

 tube be removed and filtered, and to the filtrate added carefully 

 to neutrahzation dilute alkali, a^ precipitate, insoluble in water 

 but soluble in excess of alkali (or acid), is thrown down. This 

 is in most respects like acid-albumen, but has been called para- 

 peptone. The longer digestion proceeds, the less is there of 

 this and the more of another substance, peptone, so that the 

 former is to be regarded as an intermediate product. Peptone 

 is distinguished from albuminous bodies or proteids by — 1. 

 Not being coagulable from its aqueous solutions on boiling. 

 2. Diffusing more readily through animal membranes. 3. Not 

 being precipitated by a number of reagents that usually act 

 on proteids. 



In ai'tiflcial digestion it is noticeable that much more fibrin 



