OF FOOD. 3 



Albuminoids. There exist in the animal body several substances which, 

 although close allied to the proteids, are separated from them by several well- 

 marked differences. One of these, Collagen, is of great importance, as yield- 

 ing Gelatin. It constitutes the "organic bases " of bone, and is the most 

 important constituent of the connective tissues generally. Gelatin is the pro- 

 duct of boiling "collagen." It swells in cold water, but does not dissolve ; 

 its solution in hot water "gelatinizes" on cooling. It is precipitated by 

 tannin and corrosive sublimate, not by acetic acid and potassic ferrocyanide. 

 Chondrin, a similar product obtained from cartilage, is distinguished from 

 gelatin by being precipitated by acetic acid. 



CARBOHYDRATES. 



Cellulose. The material of the outer membrane of the plant-cell is un- 

 affected by dilute acids or alkalies, but is converted by strong sulphuric acid 

 into a body which is coloured blue by iodine. 



Starch is the chief constituent of the " Starch grains " contained in the cells 

 of plants ; in these grains it is enclosed in concentric envelopes of an insoluble 

 body resembling cellulose ; it is soluble in water, the solution being opalescent ; 

 it is coloured blue by iodine ; the blue colour disappears on heating, but re- 

 appears on cooling, unless the heating has been long continued. Starch is 

 converted by prolonged action of weak acids into Dextrin (a body which is 

 coloured red by iodine), and eventually into grape sugar. It is converted in 

 presence of diastatic ferments, e.g., those of the salivary glands, pancreas, and 

 liver into grape sugar. 



Grape Sugar or Dextrose or Starch Sugar (C 6 H 12 O 6 ) occurs in extremely 

 small quantity in blood, muscle, and other tissues, and (according to Briicke) 

 in normal urine. It is distinguished by its power of reducing certain metallic 

 salts, by its dextro-rotatory action, by its splitting, when subjected to the 

 action of the yeast-plant at a suitable temperature, into alcohol and carbonic 

 acid, and by its yielding lactic acid in presence of albuminous bodies in process 

 of putrefaction. It is soluble in \\ parts of cold water, and to any extent in 

 boiling water. From its solution in boiling alcohol, it readily crystallizes. 

 On the power of reduction possessed by grape sugar depends the important 

 test known as "Trommer's Test," which consists in adding solution of 

 potassic hydrate to the liquid supposed to contain sugar, and then weak 

 solution of cupric sulphate, so long as the precipitate of cupric hydrate first 

 formed redissolves on agitation. On gently heating, a yellow precipitate is 

 formed of cuprous hydrate, or a deposit of cuprous oxide (see Practical Part, 

 Section II.) 



Milk Sugar or Lactose constitutes about 15 percent, of the solids of milk : 

 it can be obtained directly from whey, after separation of albuminous com- 

 pounds, by crystallization in rhombic prisms (C I2 H 22 O n -f- H 2 O) : it is con- 

 verted by the prolonged boiling with weak acids into fermentescible sugars 

 (galactose and dextrose) ; it is transformed very readily into lactic acids 

 under the influence of a ferment usually present in milk. 



Cane Sugar does not occur in the animal body, but is an important con- 



B 2 



