General Physiology of Animals. 343 



alone. If a rapid succession of stimuli be applied to the 

 muscle, an absolute maximum is at length reached. The 

 muscle is then said to be in tetanus. The absolute maxi- 

 mum gradually falls as the muscle becomes exhausted. 



Nervation. While the rate of muscular motion is com- 

 paratively slow the chemical changes which take place in 

 nerve-cells and nerve-fibres during nervous activity arc 

 extremely rapid. It has been calculated that a nerve 

 impulse travels along a nerve at the rate of 200 feet per 

 second. The rapidity of the metabolic processes in the 

 brain is proverbial. The essential nature of these processes 

 and of nerve motion is unknown. 



Heat. The ultimate form into which all other forms of 

 energy become resolved is manifested more abundantly and 

 can be studied more easily in the animal than in the plant. 

 The temperature of the living body in the case of the higher 

 animals varies, but the majority exhibit a uniform tempera- 

 ture of about 100 Fahr. In birds it is slightly higher. In 

 the frog, however, the temperature of the body is that of the 

 surrounding air. Hence the old-fashioned classification of 

 animals into cold-blooded (e.g. fish, frogs), and warm- 

 blooded (e.g. birds, mammals). Heat, as has been already 

 stated, is the final results of katabolic processes taking 

 place in the body ; the more work done the more heat 

 evolved, and the more food required to repair waste. With- 

 out entering into detai's on the subject of animal heat, it 

 may be of interest to point out the important relationship 

 that exists between the evolution of heat and the metabolic 

 changes of the body viewed in relation to the constant 

 variation in atmospheric temperature. In the case of a hot- 

 blooded animal if the temperature of the air decrease, more 

 heat-producing food (fats) must be oxidised to keep up the 

 balance ; if the temperature increase, less food is of course 

 needed. If, on the other hand, the animal be cold-blooded 

 and the temperature of the air decrease, metabolism is 

 diminished \ if the temperature of the air increase, meta- 



