THE VITAL UNITS CALLED CELLS 1 89 



their neighbors or by the fluids surrounding them. Some 

 by virtue of their occupations have a shorter life than others. 

 There is reason to believe that the hfe of leucocytes in the 

 blood stream is Hmited to a few days, perhaps even to a 

 few hours. The average age of red blood cells is placed at 

 between fifteen and forty days, while nerve cells usually 

 remain alive during the whole Hfe of the individual. 



During the life of any man or woman a continual replacement 

 of dead cells by Hving cells is provided for. The rate of this 

 replacement is naturally conditioned by the length of life 

 of the cells in question. Since, as we have said, the nerve 

 cells may remain functionally active as long as the individual 

 lives no arrangement is normally necessary for this replace- 

 ment, which explains the lasting injury resulting from the 

 death of nerve cells in various forms of paralysis and in 

 organic diseases of the nervous system. The expectation of 

 life of blood cells being short they are replaced relatively 

 quickly by the production of new cells, chiefly in the bone 

 marrow and lymph glands. 



But one of the most interesting reservoirs of new life (or 

 rather of more life) in the body is found in the deeper layers 

 of the epidermis. Here the cells multiply rapidly and new 

 cells are supplied from within to take the place of the dying 

 and dead cells on the surface which are continually being 

 cast off". It is a true saying that "while we are in life we are 

 in death" because this protective investment of dead cells 

 is held like a shield between the living delicate tissues 

 beneath and the environment outside. It is a kind of shock 

 absorber. 



This insulation, however, if it were complete, would make 

 us totally inert and unresponsive. We may regard the cells 

 of our sense organs as sentinals looking out through specially 

 constructed windows in our skin so that they may perceive 

 what occurs without, and signal the results to the entire 

 body through the activity of certain nerve cells, devoted to 

 conduction, which group themselves in series like the relay 

 stations in a telegraph system. The analogy is a close one, for, 

 with the passage of each nervous impulse, slight electrical 

 variations take place. Happily for our peace of mind the 

 receptive cells are only attuned to certain changes in our 



