FOOD DIGESTION AND RESPIRATION 71 



which is a condensation of several molecules of glucose, but less 

 than- the number contained in the molecule of starch, together 

 with the sugar called maltose, consisting of two molecules of 

 glucose united by the elimination of H.^O, and is therefore one of 

 the di-saccharides, or more correctly, bi-hexoses. 



After the food is swallowed, it passes quickly down the 

 oesophagus, or gullet, and reaches the stomach. This is a large 

 bag in the course of the alimentary canal, and has muscular rings 

 at both ends, so that the food is not allowed to leave it until it 

 has been thoroughly exposed to the action of the digestive juice 

 secreted by the lining cells. This "gastric juice" is strongly acid, 

 and stops the further activity of amylase as soon as the mass in the 

 stomach has become penetrated by the acid. But this does not 

 take place very quickly, so that the salivary amylase continues its 

 action for some further time. The digestion of starch is, however, 

 not completed by the time that the food is passed on to the small 

 intestine, a long tube in which a variety of processes takes place. 

 Although the stomach is an important organ for the digestion of 

 proteins, as we shall see presently, the process is incomplete, even 

 in this case. In fact, there is no absorption into the blood of any 

 digestive products from the stomach. They are not yet in the 

 state required by the tissue cells. 



Maltose, dextrin, and unaltered starch pass, then, into the 

 small intestine, and are next subjected to the action of the 

 pancreatic juice, a mixture of powerful enzymes, one of them 

 being an amylase which completes the digestion of starch and 

 dextrin, certainly as far as maltose and probably, to some extent, 

 as far as glucose. Maltose is acted on by an enzyme, maltase, 

 which completes the hydrolysis to glucose. This enzyme is 

 produced by the cells lining the intestine. Pancreatic juice is 

 a liquid formed by a secreting gland by the side of the intestine, 

 and poured into the intestine by a duct. 



The glucose so formed passes by diffusion into the blood 

 vessels, with which the wall of the intestine is richly supplied. It 

 is carried by the blood to the liver, through which it passes before 

 reaching the rest of the body. 



Since the whole of the glucose produced by the digestion of 

 a particular meal is not wanted for use at once, a part of it is 

 stored in the liver, and also in the muscles, in the form of an 

 insoluble starch-like substance called " glycogenr The liver itself 

 contains an amylase, which changes the stored glycogen into sugar 

 when required. Moreover, what we have learned about the 

 reversible action of enzymes tells us that this same enzyme is 

 able to form glycogen from sugar, when the conditions are 

 favourable. Since the equilibrium position in such cases depends 



