134 THE CELL 



them from the fluid by which they are surrounded, whilst terres- 

 trial plants take them up with their roots from the soil, which is 

 saturated with moisture. The cells of the higher animals nourish 

 themselves by absorbing substances held in solution in fluid media, 

 which must first, by means of complicated processes, be introduced 

 by them into their bodies. These fluid media are the chyme of the 

 intestinal canal, blood, chyle, and lymph. They play the same 

 part in the economy of the animal cell as the water and moisture 

 of the earth do in that of the lower organisms and of plants. 



In opposition to the antiquated physiological view that the prin- 

 cipal metabolic processes take place in the fluids of the body, too 

 much stress cannot be laid upon the following proposition, that 

 the cells are the site of the absorption, excretion, and transforma- 

 tion of material ; the fluids only function in conveying the nutrient 

 material in a fluid condition to the cells, and in carrying away the 

 waste products. 



Between the cell and its surrounding medium, there exist the 

 most complicated physical and chemical conditions of interchange. 

 Their investigation is a most difficult undertaking, and can only 

 be entered into here to a very limited extent. 



Each cell adapts itself most closely in its organisation to the 

 surrounding medium, any considerable variation in the concentra- 

 tion or composition of which causes its death. However, in many 

 cases, great alterations may be permanently endured, provided 

 that the consecutive stages are allowed to merge slowly and gradu- 

 ally into one another, so that the cell has time to adapt itself to its 

 new conditions. 



As has been already mentioned in the chapter on chemical 

 stimuli (p. Ill), fresh-water Amoebae are .able to accustom them- 

 selves to living in salt water, whilst marine animals can adapt 

 themselves to the presence of a greater or less percentage of salt 

 in the water surrounding them. Apparently they adapt them- 

 selves by adjusting the fluid they contain to the surrounding 

 medium. It is on this account that when the changes are made 

 suddenly, death immediately ensues, the protoplasm either swelling 

 up, or shrinking and coagulating. 



Since in Vertebrates the cells which are bathed in the tissue- 

 fluids exist under such extremely complex conditions, it is difficult 

 to keep small portions of tissue alive, even for a short time, when 

 once they have been separated from the rest of the body ; for even 

 the tissue-fluids become quickly altered as soon as they are sepa- 



