142 GENERAL PHYSIOLOGY 



oxygen. Hence within certain limits living substance is fairly 

 independent of the quantity of oxj-gen that is at its disposal. 

 But all organisms without exception absolutely require for their 

 life a certain quantity of oxygen. If separated from it they 

 invariably die after a shorter or longer time. Without respiration 

 no life exists. 



Finally, all organisms without exception take in water, and with 

 it certain salts, which, in so far as they are not contained in the 

 other food, are likewise essential to the maintenance of life, 

 although wide differences prevail among the different organisms 

 as regards the kind of salts required. Salts of sodium, potassium, 

 magnesium, calcium, and iron, containing phosphorus, sulphur, 

 carbon, and chlorine, appear to be essential to all organisms. 



We have thus reviewed the food-stuffs of organisms ; we will 

 now consider how the individual cell takes in this food. 



2. The Mode of Food- Ingestion ly the Cell 



Food-stuffs exist partly in the gaseous, partly in the liquid, i.e., 

 dissolved, and partly in the solid condition ; but by no means all 

 living cells are able to take in solid food. The great majority of 

 all cells, almost all animal tissue-cells, a great number of plant- 

 cells, and many unicellular organisms take in dissolved food only, 

 the latter either primarily consisting exclusively of dissolved sub- 

 stances, or being transformed from the solid to the dissolved state 

 by the agency of certain secretions outside the cell-body. Only 

 relatively few kinds of cells are fitted for the ingestion of solid 

 food. 



The process of ingestion of gaseous and dissolved food-stuffs, which 

 is termed resorption, is essentially different, according as the 

 cells in question do or do not possess a cell-membrane. In cells 

 that do not possess a membrane all dissolved food-substances of 

 whatever kind pass directly into chemical relations with the mate- 

 rials of the living substance at the surface of the protoplasm. 

 W'here a membrane is present, it is necessary that the food-stuffs 

 have the power of diffusing through membranes. The substances 

 that cannot do this must, therefore, first be transformed into 

 diffusible substances in order to reach the interior of the cell. 



Every cell, however, is capable of ingesting gaseous and dissolved 

 food. 



In plants the carbonic acid and oxygen of the air come into 

 direct contact with the cells of the leaves. A similar arrange- 

 ment is found in the lungs of vertebrates. The finest branches 

 of the bronchial tubes end in small blind sacs, the so-called 

 pulmonary alveoli, which are formed by an extremely thin layer of 

 epithelium-cells and are surrounded by a close network of likewise 

 very thin-walled blood-capillaries. The oxygen of the air inspired 



