94 INTRODUCTION TO GENERAL PHYSIOLOGY 



change in length unless the object yields to our efforts. The fact 

 becomes obvious when we break a string by pulling it (E., p. 204). 

 Take a piece of fine, non-extensible string and find the weight 

 necessary to break it. Then take another piece and break it by 

 pulling with the muscles. No change in length of the muscles can 

 occur until the string breaks, but a force equal to the weight in the 

 previous experiment must have been exerted on the string in order 

 to break it, and this was done before the string broke and the 

 muscles shortened. 



The fact is expressed in the statement that the muscle develops 

 a state of " tension " if not allowed to shorten. This is, indeed, the 

 more fundamental fact, since it is the production of the state of 

 tension that causes the muscle to shorten and to do external work. 



If we take a coiled steel spring, hang it vertically and increase 

 its length by pulling upon it, a state of tension is produced in it, 

 and, by virtue of this, if a weight is attached to its lower end and the 

 hand pulling it is removed, the tension of the spring does work by 

 raising the weight (E., p. 204). It is somewhat difficult to realise 

 the state as applied to muscle. If we take a coil of lead wire 

 similar to the steel spring and stretch it to the same length, no 

 tension is developed, because lead has not the elastic properties of 

 steel. It may be said, then, that a muscle, when it " contracts," 

 changes its state from that of a stretched lead coil to that of a 

 stretched steel coil, without necessarily altering its length. 



The details of the way in which this happens and the origin of 

 the energy set free belong to one of the most difficult parts of 

 physiological science, and are by no means clear, as yet. 



In order that a muscle may be put into a state of activity, we 

 may apply what is called a " stimulus," either to the muscle itself 

 or to the nerve which enters it. The most convenient form of 

 stimulus is an electrical one, since it can be adjusted in strength in 

 an accurate and simple manner (E., p. 205). But other forms of 

 stimulation may be used a tap, heat, or application of salt. 



Let us make what is called a " nerve-muscle preparation " from 

 a frog (E., p. 204). We can make the muscle do work by raising a 

 weight, although, being cut out of the body, it is impossible for it 

 to receive any supply of energy from outside itself. It must, there- 

 fore, contain a store of energy within itself, and may be compared to 

 a wound-up clock spring, a raised weight used to drive some 

 mechanism, or again, the cordite charge in a cartridge. 



If, by repeated stimulation, we make an isolated muscle perform 

 a long series of contractions, we exhaust its store of potential 

 energy ; it becomes " fatigued," in one sense of the word. This 

 store of energy is not replenished under the conditions of our 

 experiment ; but we know from experience that a muscle recovers 



