GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 139 



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

 heat. 



Liberation of Mechanical Energy. — The amount of work which a muscle 

 can do depends on the following conditions : 



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

 than those of cold-blooded animals ; a human muscle can do twice the amount 

 of work of an equal amount of frog's muscle. The muscles of certain insects 

 have even greater strength. 1 Within the same animal there are great differ- 

 ences in the capacity of different forms of muscle tissue (see p. 107). Pale 

 striated muscle tissue, although more capable of rapid liberation of energy, 

 has not the endurance or the strength of the red striated muscle tissue ; and 

 different forms of non-striated muscle differ among themselves as well as 

 from striated in their capacity for work. 



(6) The condition of the muscle. Any of the influences which lessen the 

 irritability of the muscle — lack of blood, fatigue, cold, etc. — decreases the power 

 to liberate energy, and any influence which heightens the irritability is favora- 

 ble to the work. The effect of tension to heighten irritability has already been 

 referred to and is of especial interest in this connection, since the very re- 

 sistance of the weight is, within limits, a condition favorable to the liberation 

 of the energy required to overcome the resistance. This will be referred to 

 again. 



(c) The strength and character of the stimulus. The liberation of energy is, 

 up to a certain point, the greater, the stronger the excitation. Furthermore, 

 rapidly repeated excitations are much more effective than single excitations, 

 because a series of rapidly following stimuli, both by altering the irritability and 

 by inducing the form of contraction known as tetanus, act to produce powerful 

 and high contractions. Bernstein states that the energy developed by the 

 muscle increases with the increase of the rate of excitation from 10 to 50 per 

 second, at which rate the contraction power may be double that called out by a 

 single excitation. 



(d) T7ie method of contraction and the mechanical conditio)is under winch 

 the work is done. In estimating the amount of mechanical energy liberated 

 by a muscle, we observe the amount of external work which it accomplishes, 

 i. e. the amount of mechanical energy which it imparts to external 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 conies 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. An unweighted 

 muscle in contracting does no external work; a muscle, however vigorously 

 it may contract, if it be prevented from shortening, does no external work ; 

 finally, a muscle which raises a weight and then lowers it again when it 

 relaxes, does not alter its surroundings as the total result of its activity, and 



1 Hermann : Handbuch der Physiologie, 1879, Bd. i. S. 64. 



