Page:Popular Science Monthly Volume 23.djvu/708

690 the mouth by three pairs of glands to the extent of some twenty ounces a day. It consists in great part of water, with a little salt and a peculiar substance called ptyaline, which possesses the property of changing starch into sugar, the change being accomplished most completely when the starch is dissolved or baked, at a temperature of about 98 Fahr., the normal temperature of the body. Although this ptyaline is present in the saliva to the extent of only one part in five hundred, yet, on its presence and action, the heat, and consequently the life of the body, is largely dependent; hence the importance of avoiding any unnecessary waste of it, such as frequently and unnecessarily accompanies smoking. Hence, likewise, we see the importance of chewing the food slowly and thoroughly, that it may be all brought under the influence of the ptyaline; and thus we can understand how indigestion or dyspepsia may be caused by hasty chewing or by excessive spitting, the starchy portion of the food in either case lying in the stomach as an undissolved mass.

Bread-making we have already stated is a form of cooking. The heat of the oven has converted the outside of the bread into sugar, and the starch in the inside has in fact been boiled in the steam of the water which the dough contained, so that it has become capable of being readily converted into sugar. The porous nature of the bread favors this conversion; for the saliva easily penetrates through the whole of the spongy mass; and the change is still further assisted by the water which the bread contains to the extent of some forty per cent. Biscuits, on the other hand, being as a rule dry and non-spongy, are less suitable for ordinary use, although containing in the same weight far more food-material than bread.

It may surprise some of our readers to be told that the starch of bread has not the slightest nutritive property. Its sole office is a heat producer; and, just like the coal of the engine, the starch or sugar is burned up inside us to keep up the temperature of the machine. It is the gluten, the sticky, tenacious matter in the grain, which is the nutritive, flesh-forming material; but in the present article we have no space to follow the changes which it undergoes in the system, for we are simply treating of starch at present; and we trust we have made it clear how it is changed into sugar, and thus made soluble and fit for absorption into the juices which keep the body at a uniform temperature and in good repair.

It is a common but mistaken notion that sago and tapioca are very nutritious. On the contrary, they consist almost wholly of starch, with only about three per cent of gluten, so that, unless cooked with milk or eggs, they form a very insufficient food. The same is the case with Indian-corn flour and arrowroot, which have scarcely a particle of nutritious matter in them, so that it is a great mistake to feed an invalid or a child on such materials. They are no doubt useful, as easily digested heat-producers; but they must be cooked with milk or