244 DIGESTION 



The enzymes are, so far as we know, formed in the glands themselves. 

 During the intervals between meals they are deposited in the glands, to be 

 poured out when required for digestion. But we do not find the finished 

 product in the glands : instead, precursors of the enzymes, the so-called zymo- 

 gens, are elaborated in them and are transformed into the enzymes either 

 during the act of secretion or in the secretory product after it is given off 

 (Fig. 99). 



Artificial digestion is often employed in studying the action of the digestive 

 fluids on the foodstuffs i. e., a given food is mixed either with the fluid secreted 

 by a gland or with an extract of the gland, and the mixture is kept for a time 



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FIG. 99. Transverse section of the hepato-pancreas of an isopod crustacean, fixed in osmic 

 acid, showing the gradual transformation of zymogen granules into secretion droplets, 

 after Murlin. The zymogen granules appear first immediately about the nucleus (Zym'y.). 

 As the cell grows in size the granules increase in number so as to almost fill the cell (Y. zym.). 

 Still later they absorb fluid from the protoplasm and swell up, being finally discharged 

 from the free border of the cell as secretion droplets (M. zym.). Frag., fragment of a cell broken 

 off and lying in the lumen of the gland ; Nuc., nucleus of a mature cell. 



at body temperature. A great mass of important facts has been obtained in 

 this way; but we cannot apply the results of artificial digestion to the natural 

 process in the body without some reservation. For, even neglecting the me- 

 chanical work of the alimentary canal, there are several important differences 

 between the two. (1) In natural digestion the fluid is always adapted in quan- 

 tity and quality to the quantity and character of the food acted upon, while in 

 artificial digestion both the quantity and the character are fixed for a given 

 experiment. (2) In the natural process the products of digestion are removed 

 by absorption as soon as they are formed; in artificial digestion they remain in 

 the mixture. This is of no small consequence; for in the one case the course of 

 digestion is affected by the presence of these products (cf. page 38), while in 

 the other this is prevented by their prompt removal from the sphere of action. 

 (3) Finally, in the normal course of digestion the different secretions may influ- 

 ence each other so that the final result may be essentially modified. 



