1 66 ANATOMY AND PHYSIOLOGY 



The nature of the changes which the food must undergo is, a separation 

 or splitting into simpler bodies. Most food substances are complex and 

 insoluble. The object of digestion is to convert them into simple and soluble 

 substances which can be absorbed. In order to accomplish this, they must 

 be not only mixed with water, which is a mechanical process, but combined 

 with it or hydrolysed which is a chemical process. The digestive enzymes 

 belong to the class called hydrolytic enzymes because they act by hydrolysis. 



Hydrolysis means decomposing the water which is present and uniting its 

 elements, H and O, with other substances (also in process of decomposition or 

 breaking down by the action of the same enzyme). 



According to Hammarsten, no glands in the body can work 

 so rapidly, can produce so great a quantity of fluid in the same 

 time as the salivary glands, not even the kidneys. Eight to four- 

 teen times the weight of all the glands together may be produced 

 within one hour. 



Saliva has the power of splitting sulphureted hydrogen from 

 the sulphur oils of onions, radishes, etc. 



Clinical note. Ptyalin exists in the saliva at birth but does not become 

 active under three or four months of age. 



SUMMARY 



Digestion.- Is the process of so changing the food that it may 

 be absorbed. 



Absorption.' Is the process of taking up certain substances 

 and conveying them to, the blood. 



Circulation.' Is the process of carrying the blood and other 

 substances to every part of the body. 



Assimilation.' Is the process which goes on in the tissue cells 

 whereby they make use of the food which is conveyed to them. 



We have now to study the organs which distribute the products 

 of digestion, and the composition of the food-bearing fluids 

 blood and lymph. Assimilation is nature's own secret, not yet 

 revealed to the mind of man. This is a phase of metabolism. 



ABSORPTION OF FOOD 



Accompanying the digestion of food the absorption of nutritive 

 principles takes place. 



It is quite possible that some portion of the sugars is taken up 

 by blood-vessels of the stomach, and it is probable that more or less 



