122 The Vertebrate Organ Systems 



the increased demands of the active muscles. As exercise continues, 

 the adjustment occurs. Such an adjustment results in the so-called 

 "second wind." 



Thermal Changes. — Xot all the energy released during the series 

 of chemical reactions just described goes into work; a greater part 

 of it actually is lost as heat. Both the anaerobic and aerobic phases of 

 muscle activity produce heat. The first or initial heat occurs imme- 

 diately upon contraction, while the aerobic phase or delayed heat occurs 

 during muscle recovery. 



All the heat produced by muscle activity is not wasted; much of 

 it is important for the maintenance of the constant body temperature. 

 Shivering is a muscular response to lowered body surface temperature. 



Muscles are comparatively efficient in the work produced from the 

 amount of energy expended. It has been estimated that the efficiency 

 is from 18 to 21 per cent. As contrast, a steam engine is considered 

 to be from 9 to 19 per cent efficient. 



Physical Changes. — During contraction, the individual muscle 

 shortens. While the volume of the muscle remains constant, the shape 

 is considerably altered. This shortening of the entire muscle is, of 

 course, due to the shortening of the many fibers composing it. 



Electrical Changes. — During the muscle contraction, certain elec- 

 trical changes occur. Each contraction results in a change in the electric 

 potential which produces a measurable flow of electrical current. 



The Simple Muscle Twitch. — The study of the mechanical changes 

 which occur during muscular contraction has been facilitated by the 

 fact that single muscles may be removed intact and observed. A muscle 

 commonly used for this is the gastrocnemius muscle of the hind leg of 

 the frog. This is easily removed with its attached sciatic nerve. 



For these observations, a special instrument, the kymograph (Fig. 

 36), is utilized. This is a rotating drum which moves at a constant 

 speed that can be regulated. The muscle is mounted with a portion 

 of the femur in a special clamp, and the lower portion is attached under 

 some tension to a stylus. This stylus records the muscular contractions 

 on a smoked paper placed on the drum of the kymograph. As the 

 drum revolves, the movements are traced. The time relationships can 

 be noted because the drum is rotating at a known speed. 



Once this setup is completed, the single muscular contraction or 

 "twitch" can be studied. A single electrical shock is applied to the 



