MUSCULAE WORK. 



489 



Nevertheless, the difference is so small that, when a muscle is caused to contract 

 by stimulation of its motor nerve, practically the whole muscle appears to contract 

 simultaneously and at once. 



4. A complete, uniform, momentary contraction of all the fibres of a muscle can 

 only take place when all the fibres are excited at the same moment. This occurs 

 when the electrodes are placed at both ends of the muscle, and an electrical 

 stimulus of momentary duration passes through the whole length of the muscle. 



300. MUSCULAR WORK. Muscles are most perfect machines, not only 

 because they make the most thorough use of the substances on which their activity 

 depends ( 217), but they are distinguished from all machines of human manufac- 

 ture by the fact that, by frequent exercise they become stronger, and are thereby 

 capable of accomplishing more work (Du Bois-Reymond). 



The amount of work (W) which a muscle can perform is equal to the product 

 of the weight lifted (p) and the height to which it is lifted (h), i.e., W = ph 

 (Introduction). Hence, it follows that when a muscle is not loaded (where p = 0), 

 then w must be = 0, i.e., no work is performed. If, again, it be overloaded with 

 too great a load, so that it is unable to contract (h = 0), here also the work is nil. 

 Between these two extremes an active muscle is capable of doing a certain amount 

 of "work." 



I. Work with Maximal Stimulation. When the strongest possible, or maximal 

 stimulus is applied i.e., when the strength of the stimulus is such as to cause a 

 muscle to contract to the greatest possible extent of which it is capable, the amount 

 of work done increases more and more as the weight is increased, but only up to a 

 certain maximum. If the weight be gradually increased, so that it is lifted to a 

 less height, the amount of work diminishes more and more, and gradually falls to 

 be = 0, when the weight is not lifted at all. 



Example of the work done by a frog's muscle (Ed. Weber) : 



160 



[Suppose a muscle be loaded with a certain number of grammes, and then caused to contract, 

 we get a certain height of contraction. Fig. 338 shows the result of an experiment of this 

 kind. The vertical lines represent the height to which 

 the weights (in grammes) noted under them were raised, 1 

 so that, as a rule, as the weight increases the height to | 

 which it is raised decreases.] 



Laws of Muscular Work. 1. A muscle can 

 lift a greater load the larger its transverse section, 

 i.e., the more fibres it contains arranged parallel 

 to each other. 



2. The longer the muscle, the higher it can lift 

 a weight. 



3. When a muscle begins to contract, it can lift 

 the largest load j as the contraction proceeds, it < 



can only lift a less and less load, and when it is . . , .. , 



, ., J . i . i i i. 1 Height to which each of the weights 



at its maximum 01 shortening, only relatively very y ra i se( j t 



light loads. 



4. By the term " absolute muscular force " is meant, according to Ed. Weber, 

 just the weight which a muscle undergoing maximal stimulation is no longer able 

 to lift (the muscle being in its normal resting phase), and without the muscle at 

 the moment of stimulation being elongated by the weight. 



150 



250 

 grammes. 



Fig. 338. 



