GENERAL PHYSIOLOGY OF MUSCULAR TISSUE. 53 



A voluntary contraction in man may be regarded as a state of tetanus, 

 for if the curve of a voluntary movement be examined, it will be found to 

 consist of intermittent vibrations. The simplest voluntary movement of a 

 muscle, however rapidly it may take place, lasts longer than a single muscular 

 contraction due to an induction shock. The most rapid voluntary contrac- 

 tion is the result of from 2.5 to 4 stimulations a second, and has a duration 

 of from 0.041 to 0.064 f a second. A continuous voluntary contraction is 

 an incomplete tetanus. The number of stimuli sent to the muscle is on the 

 average, 16 to 18 for rapid contractions, 8 to 12 for slow contractions. 



The Production of Heat and Its Relation to Mechanical Work. 



The transformation of energy which takes place during a muscle contraction, 

 and which is dependent upon chemic changes occurring at that time, mani- 

 fests itself as heat and mechanical work. While heat is being evolved con- 

 tinuously during the passive condition of muscles, the amount of heat is 

 largely increased during general muscle contraction. A skeletal muscle of 

 a frog e. g., the gastrocnemius when removed from the body, shows, after 

 tetanization, an increase in its temperature of from 0.14 to 0.18 C., and 

 after a single contraction of from 0.001 to 0.005 C. While every muscular 

 contraction is attended by an incre:' se in heat production, the amount so pro- 

 duced will vary in accordance with certain conditions e. g., tension, work 

 done, fatigue, circulation of blood, etc. 



Tension. The greater the tension of a muscle, the greater, other conditions 

 being equal, is the amouunt.of heat evolved. When the ends of a muscle 

 are fastened so that no shortening is possible during stimulation, the maximum 

 of heat production is reached. In the tetanic state the great increase of 

 temperature is due to the tension of antagonistic and strongly contracted 

 muscles. The evolution of heat, therefore, bears a relation to the resistance 

 against which the muscle is acting. 



Mechanical Work. If a muscle contracts, loaded by a weight just sufficient 

 to elongate it to its original length, heat is evolved, but no mechanical work 

 is done, all the energy liberated manifesting itself as heat. When the weight 

 which has been lifted is removed from the muscle at the height of contraction, 

 external work is done. In this case the amount of heat liberated is less, 

 owing to the work done, for some of the heat generated is transformed into 

 mechanical motion. According to the law of the conservation of energy, 

 the amount of heat disappearing should correspond in heat units to the 

 number of foot-pounds produced by muscular contraction. 



Muscle Sound. Providing a muscle be kept in a state of tension during 

 its contraction, the intermittent variations of its tension cause the muscle 

 to emit an audible sound. If the muscle be tetanized by induction shocks, 



