746 APPENDIX. 



divided into two classes, called (1) organized, and (2) unorganized or 

 soluble. 



(1.) Of the organized, yeast may be taken as an example. Its activity 

 depends upon the vitality of the yeast cell, and disappears as soon as the 

 cell dies, neither can any substance be obtained from the yeast by means 

 of precipitation with alcohol or in any other way which has the power of 

 exciting the ordinary change produced by yeast. The action of micro- 

 organisms in the alimentary canal and elsewhere is also an example of 

 the same nature. 



(2.) Unorganized or soluble ferments are those which are found in 

 secretions of glands, or are produced by chemical changes in animal or 

 vegetable cells in general; when isolated they are colorless, tasteless, 

 amorphous solids soluble in water and glycerin, and precipitated from 

 the aqueous solutions by alcohol and acetate of lead. Chemically many 

 of these are said to contain nitrogen. 



Mode of action. Without going into the theories of how these unor- 

 ganized ferments act, it will suffice to mention that: 



(1.) Their activity does not depend upon the actual amount of the 

 ferment present. (2.) That the activity is destroyed by high tempera- 

 ture, and various concentrated chemical reagents, but increased by mode- 

 rate heat, about 40 C., and by weak solutions of either an acid or alka- 

 line fluid. (3.) The ferments themselves appear to undergo no change 

 in their own composition, and waste very slightly during the process. 



Varieties. The chief classes of unorganized ferments are: 



(1.) Amylolytic, which possess the property of converting starch 

 into glucose. They add a molecule of water, and may be called hydro- 

 lytic. The probable reaction is given, p. 234. 



The principal amylolytic ferments are Ptyalin, found in the saliva, 

 and a ferment, probably distinct, in the pancreatic juice, called Amy- 

 lopsin. These both act in an alkaline medium. Amylolytic ferments 

 have been found in the blood and elsewhere. 



(2.) Proteolytic convert proteids into peptones. The nature of their 

 action is probably hydrolytic. The proteolytic ferments of the body are 

 called Pepsin, acting in an acid medium from the gastric juice. Trypsin, 

 acting in an alkaline medium from the pancreatic juice. The Succus 

 entericus is said to contain a third such ferment. 



(3.) Inversive, which convert cane sugar or saccharose into grape 

 sugar or glucose? Such a ferment was found by Claude Bernard in the 

 Succus entericus; and probably exists also in the stomach mucus. 



(4.) Ferments which act upon fats, Such a body, called Steap- 

 sin, has been found in pancreatic juice. 



(5. ) Milk-curdling ferments. It has been long known that rennet, 

 a decoction of the fourth stomach of a calf, in brine, possessed the power 

 of curdling milk. This power does not depend upon the acidity of the 



