344 Elementary Biology. 



holism is also increased. It will be seen that this relation 

 depends entirely on the fact that the temperature of the 

 warm-blooded animal is nearly constant, while that of the 

 cold-blooded animal varies with the surrounding tempera- 

 ture within certain limits. 



Light. Many animals have the power of giving out 

 luminous rays, which serve either to frighten away enemies 

 or to guide them in finding their prey in the dark. Many 

 fish, for instance, possess this power. This luminosity is 

 known as phosphorescence (p. 209). 



Electricity. The existence of normal electrical currents 

 in certain animal tissues, notably, muscle and nerve, has 

 already been alluded to. These currents are particularly 

 noticeable if the muscle or nerve be injured in any way. 

 Further, a special electrical current, the so-called nega 

 tive variation, is developed in a muscle or nerve on the 

 application of a stimulus, and this electrical wave is the fort- 

 runner of the actual contraction, or of the nervous wave, 

 and shows that certain metabolic changes are taking place 

 preparatory to the actual contraction or the passage of the 

 nervous impulse. The whole subject is, however, still under 

 consideration, for the connection between the metabolic 

 changes in the tissues and the electric currents is by no 

 means clear. 



