HANDBOOK OF PHYSIOLOGY, 



to heat also develops on the surface certain empyreumatic bodies, which 

 are agreeable both to the taste and smell. By placing meat in hot water, 

 the external coating of albumen is coagulated, and very little, if any, of 

 the constituents of the meat are lost afterwards if boiling be prolonged; 

 but if the constituents of the meat are to be extracted, it should be ex- 

 posed to prolonged simmering at a much lower temperature, and the 

 "broth" will then contain the gelatin and extractive matters of the 

 meat, as well as a certain amount of albumen. The addition of salt will 

 help to extract myosin. 



The effect of boiling upon (Z) an egg is to coagulate the albumen, and 

 this helps to render the article of food more suitable for adult dietary. 

 Upon (c) milk, the effect of heat is to produce a scum composed of al- 

 bumen and a little casein (the greater part of the casein being uncoag- 

 ulated) with some fat. Upon (d) vegetables, the cooking produces the 

 necessary effect of rendering them softer, so that they can be more 

 readily broken up in the mouth; it also causes the starch grains to swell 

 up and burst, and so aids the digestive fluids in penetrating into their 

 substance. The albuminous matters are coagulated, and the gummy, 

 saccharine and saline matters are removed. The conversion of flour into 

 dough is effected by mixing it with water, and adding a little salt and a 

 certain amount of yeast. Yeast consists of the cells of an organized 

 ferment ( Torula cerevisice), and it is by -the growth of this plant, which 

 lives upon the sugar produced from the starch of the flour, that a quan- 

 tity of carbonic acid gas and alcohol is formed. By means of the former 

 the dough rises. Another method of making dough consists in mixing 

 the flour with water containing a large quantity of carbonic acid gas in 

 solution. 



By the action of heat during baking (d) the dough continues to ex- 

 pand, and the gluten being coagulated, the bread sets as a permanently 

 vesiculated mass. 



I. Effects of an insufficent diet. 



Hunger and Thirst. The sensation of hunger is manifested in con- 

 sequence of deficiency of food supplied to the system. The mind refers 

 the sensation to the stomach; yet since the sensation is relieved by the 

 introduction of food either into the stomach itself, or into the blood 

 through other channels than the stomach, it would appear not to depend 

 on the state of the stomach alone. This view is confirmed by the fact, 

 that the division of both pneumogastric nerves, which are the principal 

 channels by which the brain is cognizant of the condition of the stomach, 

 does not appear to allay the sensations of hunger. But that the stomach 

 has some share in this sensation is proved by the relief afforded, though 

 only temporarily, by the introduction of even non-alimentary substances 



