The digestion of carbohydrates takes place chiefly in the mouth and small intestine, although from what has been said it will be inferred that only cooked starches have much chance of being acted upon in the mouth. The action of saliva is really twofold: (a) a physical one, moistening, lubricating, and dissolving soluble substances - in particular starch, which is first converted into what is called "soluble starch," a material only less colloidal than starch itself, although capable of being coloured blue by iodine; (b) chemical, due to the diastatic enzyme ptyalin, which attacks the starch and converts it by hydrolysis into erythro-dextrin, then achroo-dextrin, iso-maltose, and finally maltose. The same action takes place, though more slowly, on glycogen, and it is best carried out in a neutral medium, a very small amount of-acid in particular inhibiting it. Probably this is all the digestive action which takes place in the mouth, although some authorities claim that the saliva is able to convert 1 per cent. of starch into dextrose.

The importance of cooking carbohydrate food will now be manifest. The process of baking converts the starch of flour in varying quantities into dextrin, and careful people continue and extend this action by a prolonged exposure of slices of bread to a toasting procedure, the result being known as zwieback, or, when the soft central portion only is used, pulled bread. It is claimed by the proprietors of a considerable number of breakfast foods composed of wheat, barley, and oats that a similar effect has taken place in these products. But in any case careful mastication and insalivation will convert starch which is not absolutely raw into dextrin, and through the subsequent stages to maltose.

The secretion of saliva is a reflex action, the sight, smell, and taste of food exciting the glosso-pharyngeal and olfactory nerve endings, and so a communication reaches the chorda tympani supplying the submaxillary and sublingual glands and a branch of the glosso-pharyngeal supplying the parotid. Pavlov has demonstrated that there is not a true psychic excitation of saliva in the manner so well known in connection with gastric juice, but that it is more in the nature of a reflex action, a real physical stimulation of nerve endings just mentioned. His experiments further clearly indicate that the physical condition of the food, especially the property of "drvness," has more to do with the secretion of saliva than the nature of the food itself. Raw meat creates in a dog a secretion, but finely powdered dry flesh is at once responsible for an abundant flow. The chewing of fresh, moist bread produces very little secretion, while dry bread causes the saliva to flow in large quantities. Although acids and alkalis stimulate a flow, neither water nor physiological saline solution excites any secretion. Quartz pebbles, if clean, have a negative effect, whereas sand immediately stimulates a flow. The drier and harder the food, the greater the secretion poured out by the mucous glands - i.e., the submaxillary and sublingual - although milk, for which more saliva is poured out than for flesh, forms a striking exception to this rule. He concludes, therefore, that substances entering the mouth start a secretion of saliva solely because they excite definite physiological sensitivities.

After swallowing, the digestive action of the saliva continues in the fundus of the stomach for quite half an hour or more, until, indeed, the alkali in the saliva is neutralised by the increasing amount of free hydrochloric acid. The presence of 01 per cent. HC1 in a test-tube terminates its activity, but some time must elapse ere this percentage of HC1 can arise in the stomach, and quite an appreciable time after neutralisation has taken place on the outside of the mass of food in the stomach the ptyalin may be quite active in the central portion. It is asserted by some that the ptyalin is not actually destroyed, but only inhibited in the stomach, and Rogers and Simon claim that they have demonstrated that on reaching the duodenum it is reactivated in the presence of the alkaline pancreatic juice. Whatever truth may be in this assertion, it is clear that after neutralisation of the alkali of the saliva no further digestion of the carbohydrates takes place until the duodenum is reached and the amylolytic action of the pancreatic fluid comes into operation. An exception to this statement must, however, be made in favour of the inversion of cane sugar into dextrose and laevulose by the acid of the gastric juice, frequently aided by inverting enzymes contained in the vegetable food which has been swallowed.

Amylopsin - the diastatic ferment of the pancreatic fluid - is its most powerful constituent, acting rapidly on any cooked starch which may have escaped the saliva, and also on the uncooked starch so far as it can reach the latter through its cellulose envelope. This completes the conversion of starch into maltose.

Cellulose is practically indigestible; at least no enzyme has yet been discovered in man which is capable of attacking it, although in the lower animals cytase exerts a hydrolytic action upon it. Still, in man bacterial action in the colon in some cases exercises an influence in effecting its disintegration, although fortunately in most cases it is left intact to accomplish its function of regulating intestinal movement.

The sole object of carbohydrate digestion is to convert the more complex members of the series into one of the mono saccharoses, viz., glucose, the form most favourable for absorption. We have now, therefore, a mixture probably of maltose, cane sugar, lactose, and laevulose. The cane sugar is next attacked by the invertase of the succus entericus being split into equal parts of dextrose and laevulose, thus: -

CI2H22O11 + H2O = C6H42O6 + C6HI2O6

Cane sugar. Dextrose. Leevulose.

By a similar process lactase converts lactose into dextrose and galactose, and maltase acts on the maltose, converting it into two molecules of dextrose. Pavlov states that lactase is always present in the young of mammalian animals, not in those of other species, and that it is absent in adult mammals unless milk-sugar be added to the dietary, when by some special adaptive function the economy acquires the power of producing this ferment. A great deal of doubt has, however, been cast upon the possibility of this specialisation of function, and Aders Plimmer, amongst others, has proved that neither the pancreas nor the intestines of animals can be made to adapt themselves to any particular diet. We have already noted that in the case of the saliva the texture more than the composition of the food is the determining factor in producing the secretory response.

Thus by hydrolysis all the starches and sugars, at least those containing six carbon atoms, termed the hexoses, which alone we have been considering, are converted into mono-saccharoses. Other forms of sugar with varying carbon atoms are in existence, but the only group of practical importance are the pentoses, which are in a great measure excreted unchanged in the urine.