130 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



or motion. The proportion in which these two forms of energy shall be pro- 

 duced by a muscle may vary within wide limits, according to the state of the 

 muscle and the conditions under which the work is done. Fick l states that 

 if the muscle works against a very heavy weight, possibly ^ of the liberated 

 energy may be obtained as mechanical work, but if the weight be light not 

 more than ^ of the chemical energy is given off in this form, the muscle 

 working no more economically than a steam engine. The fact that always a 

 part, and often the whole, of the mechanical energy developed by the muscle 

 is converted to thermal energy within the muscle, and leaves it as heat, makes 

 it the more difficult to determine in what proportion these two forms of 

 energy were originally produced. Moreover, if Engelmann's view be correct, 

 that the change of form exhibited by the muscle is the result of the imbibition 

 of the fluid of the isotropic substance by the anisotropic material, this change 

 being brought about by the heat which is liberated within the muscle, we must 

 consider potential energy to be set free first as heat, a part of which is after- 

 ward changed to mechanical energy, which in part, at least, is again changed 

 to heat. 



Liberation of Mechanical Energy. In estimating the amount of mechanical 

 energy liberated by a muscle, we observe the amount of physical work which 

 it accomplishes, i. e. the amount of mechanical energy which it imparts to ex- 

 ternal objects. If a muscle by contracting raises a weight, it gives energy to 

 the weight, the amount being exactly that which the weight in falling through 

 the distance which it was raised by the muscle can impart as motion, heat, etc., 

 to the objects with which it comes in contact. The measure of the mechanical 

 work done by the contracting muscle is the product of the weight into the height 

 to which it is lifted. For example, if a muscle raises a weight of 5 grams, 

 10 millimeters, it does 50 grammillimeters of work. 



The amount of work which a muscle can do depends on the following con- 

 ditions : 



(a) The kind of muscle. The muscles of warm-blooded animals are stronger 

 than those of cold-blooded animals ; a human muscle can do two to three 

 times the amount of work of an equal amount of frog's muscle. The muscles 

 of certain insects have even greater strength. 2 



(6) The quantity of muscle-substance and the arrangement of the fibres. The 

 power of a muscle to do mechanical work, the absolute muscular force, is esti- 

 mated by the weight which, brought upon the muscle at the instant it begins to 

 contract, prevents it from shortening but does not stretch it, i. e. one which ex- 

 actly balances the contractile force of the muscle when it is excited to a maxi- 

 mal tetanic contraction. It is evident that the amount of force which can be 

 developed will depend on the amount of contractile substance and on the 

 arrangement of the fibres. Since the force which can be developed by a contract- 

 ing muscle depends largely on the arrangement of the microscopic contractile 

 mechanisms of which it is composed, it is found best, for purposes of compari- 



1 Fick : Pftuger's Archiv, 1878, xvi. p. 85. 



2 Hermann : Handbuch der Physiologic, 1879, Bd. i. p. 64. 



