690 THE POPULAR SCIENCE MONTHLY. 



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 pecul- 

 iar 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 pres- 

 ent 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 unneces- 

 sary 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 dys- 

 pepsia 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 nu- 

 tritive, 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 tempera- 

 ture 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, sx> 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 



