Ii6 SCIENCE PRIMERS. [§ vii. 



outside your body. It can only be said to be inside 

 your body when it gets into your blood. 



In the things we eat, moreover, these food-stuffs are 

 mixed up with a great many things that are not food- 

 stuffs at all ; they are packed away in all manner of 

 little cases, which are for the most part no more good 

 for eating than the boxes or paper in which the sweet- 

 meats you buy are wrapped up. The food-stuffs have 

 to be dissolved out of these boxes and packing. 



The juices secreted by the glands of which 

 we have been speaking, dissolve the food-stuffs out of 

 their wrappings, act upon them so as to make them 

 fit to pass into the blood, and leave all the wrappings 

 as useless stuff which passes out of the alimentary 

 canal without entering into the blood, and therefore 

 without really forming part of the body at all. 



This preparation and dissolving of food-stuffs is 

 called digestion. * 



Different food-stuffs are acted upon in different 

 psirts of the alimentary canal. 



The saliva of the mouth has a wonderful power of 

 changing starch into sugar. If you take a mouthful 

 of boiled starch, which is thick, sticky, pasty, and 

 tasteless, and hold it in your mouth for a few moments, 

 it will become thin and watery, and will taste quite 

 sweet, because the starch has been changed into sugar. 

 Now sugar, as you know, will readily pass through 

 membranes, though starch will not. 



The gastric juice in the stomach does not act 

 much on starch, but it rapidly dissolves all 

 proteid matters. 



If you take a piece of boiled meat, put it in some 

 gastric juice and keep the mixture warm, in a very 



