HEAT 45 



magnet. A powerful beam of light is passed through this 

 hole and the movement of the shadow of the string photo- 

 graphed on a moving plate. 



What is the relation between the electrical effect and the 

 change in form? 



The rate of propagation of the electrical wave is the 

 same as that of the wave of contraction, any condition 

 which modifies the one, modifying the other in hke degree. 

 But whereas a wave of contraction is always accompanied 

 by the electrical change, it is possible to have the latter 

 without the former. The electrical change, too, occurs 

 earher than the mechanical change. During contraction, 

 therefore, two waves pass along the muscle, an electrical 

 wave followed by a mechanical wave, the electrical wave 

 being the sign of a molecular change preparatory to the 

 mechanical wave, though the latter wave itself may 

 miscarry. 



The importance of this electrical response we shall see 

 in connection with the heart. 



6. The Thermal Effects of Contraction 



We have seen that the process of contraction consists 

 primarily in the assumption of a state of tension, and that 

 this state once attained, energ)^ may be hberated as work 

 if the muscle be allowed to shorten, as heat if shortening 

 be prevented. In warm-blooded animals the energy 

 which appears as heat, so far from being wasted, is the chief 

 factor in maintaining the temperature of the body above 

 that of the environment. 



Since in isometric contraction the energy of tension is 

 all converted into one form — heat, we can by measuring 

 the heat evolved estimate the energy of tension. 



For detecting the small elevation in temperature which 

 occurs two methods are employed — the thermopile or 

 thermo-electric couple, and the alteration in electrical 

 resistance of a copper wire. 



