32 MAN, THE ANIMAL 



Such cells are devoted to support and protection. 

 In order to do this service well, they have lost 

 through disuse their power to contract or to re- 

 spond to a stimulus. These cells stand in striking 

 contrast to the blood cells that float in the plasma- 

 fluid as isolated units. Each red blood corpuscle 

 transports oxygen from the lungs to the other cells 

 that are fixed in their location in the body, and 

 carries carbon dioxide, a waste from these same 

 cells to the lungs. They constitute the internal 

 transportation system; they always travel the 

 same route and always carry the same load until 

 exhausted, when they are caught in the spleen, the 

 cemetery for red blood corpuscles. Blood and 

 bone cells offer interesting contrast between closely 

 related cells, as both belong to the same tissue 

 system, the connective tissue : cells that are more 

 closely related than are tinners and plumbers in 

 our social system. 



The relations between this unit in muscles and 

 nerves is equally illuminating. The cell which is 

 specialized into a muscle cell has undergone a high 

 degree of development in that it contains specific 

 structures associated with the work of contraction. 

 As these cells work, their action is limited to a 

 single change — shortening their length and re- 

 turning to their normal linear shape. This they 

 do every time they work and there is no change or 



