CHAP, ii.] THE CONTRACTILE TISSUES. 89 



the work done will be the greater in the one which has the 

 larger bulk, which contains the greater number of cubic units. 

 In speaking therefore of the work which can be done by a muscle, 

 we may use as a standard a cubic unit of bulk, or, the specific 

 gravity of the muscle being the same, a unit of weight. 



Absolute power of a muscle. We have seen that with a given 

 weight a stimulus (induction shock) may be chosen of such a 

 strength that a contraction is only just visible. In such a case a 

 very slight increase of the weight would prevent even that 

 minimal contraction. Upon increasing the stimulus the minimal 

 contraction would reappear and vanish again upon a further 

 increase of the weight. Increasing the stimulus and weight in 

 this way we should be able to find out the weight which, with a 

 maximal stimulation, is just sufficient to prevent any visible con- 

 traction from taking place, a very slight diminution of weight at 

 once allowing a minimum contraction to make its appearance. 

 Such a weight is taken as the measure of what is called the 

 'absolute power' of the muscle; and from what has been said in the 

 previous paragraph, it is obvious that this will depend on the 

 number of fibres in, or more correctly, on the sectional area of, the 

 muscle. The absolute power of a square centimetre of a frog's 

 muscle has been in this way estimated at about -2800 to 3000 grms.: 

 of a square centimetre of human muscle at 6000 to 8000 grms. 



It may be worth while to mention in this connection the 

 following interesting fact. 



If the weight be determined which will stop a contraction 

 when applied directly the contraction begins, and also that which 

 stops any further contraction when applied at a moment when the 

 contraction is already partly accomplished, it will be found that 

 the second weight is much less than the first. It appears, in fact, 

 that the forces which cause the change in the form of the muscle 

 are at their maximum at the beginning of the shortening, and 

 thenceforwards decline until they become nothing when the short- 

 ening is complete. 



ITie work done. We learn then from the foregoing paragraphs 

 that the work done, i.e. the weight of the load multiplied into the 

 height of the lift, will depend, not only on the activity of the nerve 

 and muscle as determined by their own irritability, but also on the 

 character and mode of application of the stimulus, on the kind 

 of contraction (whether a single spasm, or a slowly repeated tetanus 

 or a rapidly repeated tetanus) on the load itself, and on the size 

 and form of the muscle. Taking the most favourable circum- 

 stances, viz. a well nourished, lively preparation, a maximum 

 stimulus causing a rapid tetanus and an appropriate load, we may 

 determine the maximum work done by a given weight, say one 

 gramme, of muscle. This in the case of the muscles of the frog 

 has been estimated at about four gram-metres for one gramme 

 of muscle. 



