GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 131 



son, to state the strength of a muscle and its capacity to do work, for the unit 

 of bulk, one cubic centimeter, or the unit of weight of muscle-substance, one 

 gram. Thus, the absolute muscular force of frog's muscle is estimated to 

 be about 3 kilograms per cubic centimeter, and of human muscle to be 8 to 10 

 kilograms per cubic centimeter. Fick states that the maximal amount of ex- 

 ternal work of which frog's muscle is capable is 1 grammeter per gram of 

 ni uscle-substance. 



(c) 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. 



(d) 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 

 isingle excitation. 



(e) The method of contraction and the mechanical conditions under which the 

 work is done. Inasmuch as mechanical work is measured by the product of 

 the weight into the height to which it is lifted, an unweighted muscle in con- 

 tracting does no work ; a muscle, however vigorously it may contract, if it be 

 prevented from shortening, does no work; finally, a muscle which raises a 

 weight and then lowers it again when it relaxes, does not alter its surround- 

 ings as the tot ' result of its activity, and hence does no work. Although no 

 .mechanical work is accomplished under these circumstances, physiological work 

 is being done, as is evidenced by the fatigue produced. Unquestionably mechani- 

 cal energy is developed within the muscle in all these cases, but it is all con- 

 verted to heat before it leaves the muscle. 



The amount of weight is an important factor in determining the extent to 

 which a muscle will shorten when excited by a given stimulus, and, therefore, 

 the quantity of work which it will accomplish. If a muscle be after-loaded, 

 i. e. if the weight be supported at the normal resting length of the muscle, and 

 the muscle be excited to a series of maximal contractions, the weight being in- 

 creased to a like amount before each of the succeeding excitations, there is, in 

 general, a gradual lessening in the height of the contractions, but the de- 

 crease in height is not proportional to the increase of the weight. The 

 decrease in the height of contractions is, as a rule, more rapid at the beginning 

 of the series than later, though at times an opposite tendency may show itself 



