PHYSIOLOGY OF MOVEMENT. 721 



demanded of a muscle, in other words, the greater the resistance to be 

 overcome, the greater will be the amount of heat production ; and, as a 

 consequence, in a contracting muscle, not only is energy liberated but 

 heat is developed, which is capable of being converted into actual energy. 

 The heat development of a contracting muscle is not only dependent 

 upon the amount of work done, but also on the tension of the muscle, 

 and the heat production reaches its maximum when the tension exerted 

 on the muscle is so great that it is not able to contract. 



Since a muscle in contracting is capable of lifting a weight, it is, 

 therefore, likewise capable of accomplishing work. The amount of 

 work will depend upon the size of the weight, upon the distance to which 

 the weight is lifted, and upon the time during which this lifting con- 

 tinues. It was found that the degree of contraction was proportionate 

 to the degree of stimulation. Therefore, the maximum amount of work 

 capable of being produced by a muscle is accomplished when the 

 maximum weight is lifted. The amount of work which a muscle may 

 perform is, therefore, equal to the product of the weight lifted and the 

 height to which it is lifted : thus, if a muscle contracts where no load is 

 present, it accomplishes no work; or, if it be loaded beyond the point at 

 which the load may be lifted, again no work is accomplished. If 

 the weight be gradually increased, even although it may be lifted, the 

 height to which that lift is accomplished becomes reduced, and, as 

 a consequence, the work diminishes. 



The amount of work which a muscle may accomplish is greater in 

 proportion to the transverse section of the muscle, for the longer 

 the muscle the greater is the shortening, and, accordingly, the higher the 

 lift. In muscles within the animal body the amount of shortening 

 which they may attain is never capable of reaching the maximum 

 obtained in a similar excised muscle. The force with which a muscle 

 contracts is greater at the commencement of contraction, and, when 

 a muscle begins to contract, it can, therefore, lift the largest load. If a 

 muscle loaded with a weight be stimulated with a rapidly interrupted 

 induction current, after the muscle has once contracted no further 

 lift is produced and no external work is evident; but if the muscle 

 sustain a weight at the height to which it raised it, the amount of work, 

 in this case the amount of energy developed by the prolonged contrac- 

 tion of the muscle, is converted into heat. 



When a muscle is stimulated with a feeble induced current and the 

 current then gradually increased in strength, it will be found that 

 the height to which the load may be lifted increases with the strength 

 of the stimulus. 



(d) The Electrical Phenomena in Muscle.— If the gastrocnemius 

 muscle of the frog be excised and its tendinous insertions cut off, and two 



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