34 MAN, — THE ANIMAL 



servience of others is one of the important facts 

 that we have to consider in this form of analysis 

 of the body of man. 



The second general condition governing the re- 

 lations of this unit in man is that we cannot sub- 

 tract any one class of cells and retain the life of 

 the whole structure. Organization has gone so 

 far in man and all of the higher forms of life, that 

 a given group of cells is unable to take up the work 

 of any of the other groups. If both kidneys are 

 destroyed, excretion cannot be carried on; or if 

 the lungs become tubercular, respiration is 

 hindered and finally prevented; we never learn to 

 hear or see in the case of permanent blindness. 



The interesting experiment in "cultivation" of 

 the biological unit outside of the body in a living 



Figure i2. Series of single heart-muscle cells which have 

 been observed to grow, beat separately, unite with one another 

 and finally beat in unison. Redrawn from Tower and Herm. 

 Am. Mus. Jour., 1916, xvi, 473. 



