CHEMICAL PEOCESSES IN CELLS. 143 



proteids of one class into those of another. It may transform casein of 

 milk into the proteids of blood and other tissues. Animal cells are, how- 

 ever, the seat also of certain synthetical processes, such as the formation 

 of haemoglobin from albumen and iron, with other inorganic matters, the 

 possible reformation of albumen from peptone, and the building of com- 

 plex albuminoids, such as mucin. All animal foods, nevertheless, orig- 

 inate in the vegetable kingdom. Even ca-rnivora are dependent on the 

 vegetable kingdom for their sustenance ; for the herbivora, feeding on 

 vegetable diet, become the prey of carnivorous animals, which are there- 

 fore dependent on the vegetable matters which serve to nourish the tissues 

 of the animals which serve as their food. In the vegetable cell albumen 

 is the end product of its chemical processes ; in the animal cell it is the 

 starting point. Albuminoids represent the main or essential type of 

 foods which must be supplied to the animal cell. When introduced into 

 the interior of cells, albuminoids undergo a progressive oxidation and 

 simplification, by which lower complex substances are formed. The 

 mode of decomposition of albuminoids, as well as of all organic bodies 

 in general, is different in different cells. This difference is seen in the 

 very first modification of the albuminous matters of food, which may be 

 converted into casein, myosin, etc., according as the resulting albuminous 

 body is destined to be a constituent of milk, muscle-cell, etc. Then, 

 again, after being deposited in cells the subsequent processes differ in 

 different cases, according to the nature of the cell-membrane or inter- 

 cellular substance, or the function of the special cells in the organism. 

 Finally, the development of the end products of the oxidation of the 

 albumen of cells differs in the cells of different tissues, though in all 

 cases the chemical processes in cells result in the formation of carbon 

 dioxide, water, and ammoniacal compounds. 



The most striking example of the products resulting from the oxi- 

 dation of proteids is the formation of fat from albumen. In adipose 

 tissue and in fatty degeneration of various organs the protoplasmic con- 

 tents of cells become replaced by oil, formed evidently at the expense of 

 the albuminoid constituents of the protoplasm. So, also, carbo-hydrates, 

 such as glycogen, may be produced from a splitting of the albuminoid 

 foods. Ir. addition to the carbo-hydrates and fats thus formed, a large 

 number of nitrogenous bodies are liberated in the oxidation of the 

 albuminous molecule, and might be termed ammonia compounds, such as 

 kreatin, uric acid, urea, etc. Such bodies are, as a rule, richer in oxygen 

 than albumen. 



The carbo-hydrates and fats are also subjected to progressive oxida- 

 tion in the animal body, and result, in the former case, in the production 

 of organic acids, such as lactic, formic, and oxalic, and in the latter in 

 the formation of fatty acids. As already several times mentioned, the 



