30 THE CELL 



B. THE INGESTION OF FOOD 



It is self-evident that the medium in which elementary organisms live 

 must contain all the nutrient substances, including oxygen and water, neces- 

 sary for their subsistence. Besides, different cells have very different require- 

 ments with respect to the chemical constitution of the medium, and these 

 peculiarities are conditioned, in part at least, upon the hereditary traits of 

 the species. 



Certain unicellular organisms are adapted for life in fresh water, others 

 for life in salt water. Most of them die if they be placed in distilled water. 

 Likewise if the chemical composition of the medium in which the cells live be 

 changed suddenly, they die; but if the change take place gradually and slowly, 

 they can adapt themselves to the altered nature of the medium and continue 

 to live. How great a change may be made before disturbances in the vital 

 activities of the cell appear, depends partly upon the nature of the cell and 

 partly upon the substance added to the medium. 



That even in the higher animals and man the properties of the cells are 

 altered by a changed composition of the lymph follows directly from observa- 

 tions on diseases characterized by chronic intoxication (e.g., alcoholism, 

 morphinism). 



In multicellular animals the cells are bathed by a fluid, the lymph, which 

 represents the medium in which they live. If the lymph is to be adapted 

 to its purpose, it must contain in the first place all the substances necessary 

 for the nourishment of the cells, and must possess also the other necessary 

 chemical and physical properties. 



Lymph is distinguished from those media in which unicellular organisms 

 live, by being inclosed within the body and by being formed essentially through 

 the activity of the cells themselves. The quantity of lymph is not unlim- 

 ited, for the supply of nutrient materials and oxygen which it contains at any 

 given time is soon used up, and their place is taken by decomposition prod- 

 ucts which are harmful to the body. But in order to maintain life in the 

 Metazoa it is necessary in the first place that the lymph shall be always of 

 normal constitution. To this end many organs of the body cooperate, each 

 being adapted for a special purpose. 



Experience teaches us that in general a special function cannot be carried 

 out by a single organ, but requires the cooperation of several. All those organs 

 which together accomplish a definite purpose are designated as an organ system 

 or apparatus. 



In order that the lymph may serve as the medium for the vital activities 

 of the cells, it must contain besides water certain combustible substances and 

 certain mineral constituents (all of which are comprehended under the name 

 foodstuffs) and oxygen. Neither the foodstuffs nor the oxygen come directly 

 into the lymph; to bring them there not less than three organ systems must 

 cooperate: namely, (1) the circulatory system, (2) the digestive system, and 

 (3) the respiratory system. 



The object of the circulatory system is to supply the lymph with nutrient 

 substances and with oxygen. The object of the digestive system is to take up 

 the foodstuffs necessary to the maintenance of the body and to change them 



