4 COMPARATIVE PHYSIOLOGY 



first sight appear a simple issue, it is really a very complex one, 

 and it is only within recent years that the work of A. V. Hill has 

 placed the question on a satisfactory basis. 



{a) Mechanical Response in Muscle.— In attempting to 

 grasp the significance of the chemical and thermal aspects of 

 contraction, it is necessary to measure the potential mechanical 

 energy of contraction. The inevitable Umitations of laboratory 

 equipment tend to give the beginner a distorted idea of the 

 significance to be attached to records of work done by a muscle 

 in Hfting a lever. Since the tension of the muscle is not the 

 same at every stage in the contraction, there are two obvious 

 difficulties in the interpretation of obsei-vations on isotonic 

 contractions : either the muscle is too heavily weighted and 

 cannot contract fully, or it is insufficiently weighted at the 

 beginning and cannot exert its maximal energy. A more 

 subtle difficulty lies in the fact that the muscle is an elastic 

 body. When a resting muscle is stretched by virtue of a load 

 it possesses potential energy like that of an extended spring ; 

 thus it does not follow that the work it may be made to perform 

 when its tension is increased during excitation is entirely the 

 result of the energy so liberated. An analogy given by A. V. 

 Hill will make this clear. Imagine a balance with two scale- 

 pans equally balanced when empty. Suppose that a weight of 

 one kilo is placed in one pan, the other being held in its original 

 position by a spring. The spring is now exerting a tension of 

 one kilo. If a small weight— say, lo grams— is placed on the 

 empty scale-pan it will sink, let us say, i cm., and the other 

 pan will be raised a corresponding height, thereby doing work 

 proportional to the product of the weight and the distance 

 through which it is raised. Clearly the lo-gram weight has 

 not contributed more than one-hundredth of this energy ; the 

 remainder is derived from the potential energy of the spring. 

 In just the same way, the energy which we get out of a weighted 

 muscle is not simply the amount of energy liberated by the 

 contractile mechanism sensu stricto. 



The way in which we may calculate the maximal amount of 

 work which the muscle is capable of doing in virtue of the 

 energy freed in contraction will present no difficulty to the 



