132 TEXT-BOOK OF PHYSIOLOGY 



and are known as paramyosinogen or myosin and myosinogen or myogen, 

 both of which are in a semi-fluid condition. The latter is four or five times 

 as abundant as the former. After death these substances undergo coagu- 

 lation and give rise to two solid substances known as myosin-nbrin and 

 myogen-fibrin. After being subjected to the cooking process, meats contain 

 the albuminoid body gelatin, a product of the transformation of the proteins 

 of the connective tissue. 



The percentage of fat contained within the meat substance is very 

 small except in mutton and pork, where it rises to 5.4 per cent, and 5.8 

 per cent, respectively. The fat-globules in these meats "are packed closely 

 between the muscle-fibers, and prevent the easy entrance of the digestive 

 fluids, and hence they are more difficult of -digestion than beef. The large 

 percentage of fat represented in the foregoing table is due to the presence 

 in the food, as eaten, of adipose tissue which is an addition, not a constituent 

 of meat. 



The carbohydrates vary from 0.5 to i per cent., and are represented 

 by glycogen. The principal inorganic salts are potassium phosphate and 

 sodium chlorid. 



Meat will vary, however, in composition, in nutritive value and in 

 energy-liberating capacity, in accordance with the region of the body from 

 which the specimen is taken. (See table, page 131.) 



Cooking, when properly done, not only makes the meat more palatable 

 and appetizing from the development of agreeable flavors, but converts 

 the connective tissue, which, in old animals especially, is tough and resisting, 

 into gelatin, thus rendering it more easy of mastication and digestion. At 

 the same time parasitic organisms, such as the embryonic forms of tenia or 

 tapeworm, and Trichina spiralis, as well as bacterial growths, which fre- 

 quently infest the bodies of animals, are destroyed and made harmless. 



Milk is the natural food of the young of all mammals, and is usually 

 regarded as typical on account of the ratio existing among its nutritive 

 principles. The analysis given above is that of cow's milk. Examined 

 microscopically, milk is seen to consist of a clear fluid, the milk plasma, 

 holding in suspension an enormous number of small, highly refractive oil- 

 globules, which measure on the average about 1 ^ 00 of an inch in diameter. 

 Each globule is supposed by some observers to be surrounded by a thin 

 albuminous envelope, which enables it to maintain the discrete form. 

 Others deny the existence of such a membrane. The chief protein con- 

 stituent of milk, caseinogen, is held in solution by the presence of phosphate 

 of lime. On the addition of acetic acid or sodium chlorid up to the point of 

 saturation the caseinogen is precipitated as such and may be collected by 

 appropriate chemic methods. When taken into the stomach, caseinogen is 

 coagulated; that is, it is changed into casein or tyrein. This change is 

 brought about by the presence in the gastric juice of a special ferment 

 termed rennin or pexin. 



The fat of milk is more or less solid at ordinary temperatures. It 

 is a combination of olein, palmitin, and stearin, with a small quantity 

 of butyrin and caproin. When milk is allowed to stand for some time, 

 the fat-globules rise to the surface and form a thick layer known as cream. 

 When subjected to the churning process, fat-globules run together and 

 form a coherent mass butter. 



