PHYSICAL PROCESSES IN CELLS. 47 



obtain access to the interior of cells, to there undergo the transformations 

 which the needs of the economy necessitate. In cells -which possess a 

 closed membrane, capillarity may also be concerned ; for there is reason 

 to suppose that the striated appearance which is seen on examining most 

 cell-membranes with a high power under the microscope is in reality due 

 to the presence of minute apertures, or canaliculi. Fluid will therefore 

 enter the pores in the membranes of cells, and so obtain access to the 

 protoplasmic cell-contents. The passage of fluids, however, through the 

 cell-membrane is not necessarily dependent on capillarity. For the 

 cell-membrane, like other organic tissues, is capable of absorbing water 

 by imbibition in the same way in which water is absorbed by gelatin, i.e, 

 by entering into its intermolecnlar spaces. The state of affairs is thus 

 similar to the conditions described in the experiment with the bladder 

 and salt. We have an organic tissue soaked with a fluid in contact with 

 a substance (protoplasm) having an affinity for that fluid greater than 

 the affinity of the membrane for the fluid; the fluid, therefore, leaves 

 the cell-membrane to enter the protoplasm by organic imbibition. 



The affinity of protoplasm for water is never satisfied during life : 

 or, in other words, the maximum amount of water capable of being 

 absorbed by cell-contents is never reached. Cells will, therefore, always 

 absorb fluid when brought into contact with it, and by so doing will 

 tend to increase in volume. As, however, the extensibility of cell- 

 membranes is in most cases very limited, the increase in volume of the 

 eelUconteuts will tend to cause filtration of the fluid contained in the 

 meshes of the protoplasm back through the cell-membrane to the 

 exterior. 



These facts which we have learned in reference to the imbibition of 

 fluids by organic tissues give but an imperfect idea of the processes of 

 imbibition which take place in living cells. Many fluids which are 

 absorbed by dead tissues are perfectly indifferent to living cells, and 

 there can be no doubt but that imbibition in living tissues is largely 

 governed by the nature of the chemical affinities caused by the chemical 

 processes continually taking place in the interior of active cells. Thus, 

 living tissues (muscles) are incapable of absorbing dilute solutions of 

 sodium salts ; the same tissues when dead will absorb it in large amounts. 

 Potassium salts, on the other hand, are rapidly absorbed by the same 

 tissue and almost instantly cause its death, though even in this case the 

 power of imbibition for the potassium salt is greatly increased after 

 death. Again, we shall find that the prolonged activity of many tis- 

 sues, especially the muscles, is manifested in the production of an acid 

 reaction in the cell-contents; under such circumstances, sodium solutions, 

 which are indifferent to these tissues at rest, will now be absorbed by 

 them. 



