30 NUTRITIONAL PHYSIOLOGY 



digestive processes. They are probably always cleavages, 

 large molecules giving rise to smaller. When the original 

 molecule is of extraordinary size, as with proteins and 

 starches, these cleavages have a serial character and a 

 number of intermediate products must accordingly be 

 formed. That is to say, the earlier products are in turn 

 subjected to digestion. Such cleavages are generally, if 

 not always, hydrolytic, that is, water enters into the 

 jeaction and its elements are found combined in the 

 products. For the simpler instances of digestion, as in 

 the case of fats and of the disaccharids, 1 we can write 

 precise chemical equations. We cannot do this with the 

 same accuracy for the starches, and we are still farther 

 from being able to express the exact manner in which the 

 protein molecule undergoes hydrolysis. Yet we have 

 sufficient evidence that the digestion is generally of a uni- 

 form type. 



Some constituents of the diet need no digestion. This 

 is the case with the mineral salts, so far as they are ab- 

 sorbed, with the simple sugars (monosaccharids), and with 

 alcohol. It is hardly necessary to say that water is also 

 ready for reception into the body fluids. The numerous 

 extractives are for the most part absorbed in the form in 

 which they are eaten. A diet entirely predigested seems 

 not to be practicable. If one were prepared it would have 

 to contain advanced decomposition products of the pro- 

 teins, which are bitter to the taste, and an amount of sugar 

 which would be cloying and subject to fermentation. 



Digestion is anticipated to some extent by changes in 

 our food which precede its actual arrival in the canal. 

 The ripening of fruits and vegetables, as well as the corre- 

 sponding processes in meat, illustrate this point. The 

 influence of cooking is not so constantly of a sort to 

 initiate digestion, yet in many instances it is so. For 



1 The following equation illustrates the hydrolysis of a disaccharid: 



C I2 H 2 Ai + H 2 -= 2C 6 H 12 6 . 



This means that one molecule of malt-sugar, reacting with one mole- 

 cule ol wa,ter, gives rise to two molecules of glucose. 



