218 HAND-BOOK OF PHYSIOLOGY. 



and fibrous connective tissue. Previous beating or bruising (as with 

 steaks and chops, or keeping (as in the case of game), renders the meat 

 more tender. Prolonged exposure to heat also develops on the surface 

 certain empyreumatic bodies, which are agreeable both to the taste and 

 smell. By placing meat into hot water, the external coating of albumen 

 is coagulated, and very little, if any, of the constituents of the meat are 

 lost afterward if boiling be prolonged, but if the constituents of the 

 meat are to be extracted, it should be exposed 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 albu- 

 men. The addition of salt will help to extract the myosin. 



The effect of boiling upon an egg coagulates the albumen, and helps 

 in rendering the article of food more suitable for adult dietary. Upon 

 milk, the eifect of heat is to produce a scum composed of serum-albumin 

 and a little casein (the greater part of the casein being uncoagulated) with 

 some fat. Upon 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 to swell up and burst, and so aids the 

 digestive fluids to penetrate into their substance. The albuminous mat- 

 ters are coagulated, and the gummy, saccharine and saline matters are 

 remove'd. The conversion of flour into bread is effected by mixing it with 

 water, a little salt and a certain amount of yeast, which consists of the 

 cells of an organized ferment (Torula cerevisice). By the growth of this 

 plant, which lives upon the sugar produced from the starch of the flour, 

 carbonic acid gas and a small amount of alcohol are formed. It is by 

 means of the former that the dough rises. Another method consists in 

 mixing the flour with water containing a large quantity of the gas in so- 

 lution. 



By the action of heat during baking the dough continues to expand, 

 and the gluten being coagulated, the bread sets as a permanently vesicu- 

 lated mass. 



I. EFFECTS OF AN INSUFFICIENT DIET. 



Hunger and Thirst. The sensation of hunger is manifested in 

 consequence of deficiency of food in the system. The mind refers the 

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

 duction of food either into the stomach itself, or into the blood through 

 other channels than the stomach, it would appear riot 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 



