LIVING SUBSTANCE 5 



of blood vessels and other organs have been cut out 

 and later replaced in the same or other animals 

 without injury. By keeping 'such a tissue at a 

 trifle above freezing point the period of suspended 

 vitality may be extended to weeks or months. 1 

 Recent experimenters have shown that not only may 

 the pieces of excised tissue be kept passively alive, 

 but that under proper conditions they will sprout 

 and grow like so many plant cuttings. It is only 

 necessary that they be surrounded by a nutritive 

 medium drawn from the same animal from which 

 they came, and that they be kept free from all bac- 

 teria. 



If the turtle heart in the experiment described 

 should after a while cease to beat, but later begin 

 to do so again, we would of course say that, like the 

 excised tissues just described, it was still alive during 

 its period of inactivity, although our only knowledge 

 of its being alive is derived from its subsequent 

 beating. For, we say, our idea of life, however 

 vague it may be, does not admit of discontinuity. 

 Once alive, always alive, until dead. 



The experimental physiologist is not so sure of 

 that. Such a suspended heart muscle of the turtle 

 will not beat except in the presence of salt (or some 

 sodium compound). But in pure salt solution it 

 stops beating. The pure salt acts as a poison. We 

 might now consider the muscle dead, were it not 



1 We find an analogous instance in Nature in the fact that many 

 seeds will retain their vitality unimpaired for years, until proper condi- 

 tions of warmth and moisture cause them to sprout and grow. 



