THE MECHANICAL CHANGES OF MUSCLE 201 



used simply for after- loading the muscle so that the weight shall not 

 act upon the muscle until it begins to contract. The stop. So may be regu- 

 lated so that it suddenly checks the movement of the lever at any desired 

 height above the base line. We may thus get a series of contractions such 

 as those shown in Fig. 60. It will be seen that at the points x', x", and x"' 

 the muscle was still pulling on the lever, and therefore held it up against 

 the stop. At the point A the arrested twitch returns rapidly to the base 

 line, showing that the movement of the lever in the unarrested curve above 

 this point was due to the inertia of the moving parts and not to the actual pull 

 of the muscle. In this case the period of contractile stress was about 0'02 

 seconds. 



THE ENERGY OF CONTRACTION. When a muscle contracts we may 

 nceive of it as converted into a body with elastic properties other than 



hose which it possesses during rest. Directly after it has been excited 

 t possesses potential energy which can be measured by the isometric method 

 tension and which will degenerate in a few hundredths of a second into 



eat, or can be turned into work by allowing the muscle to shorten and to 

 raise a weight, as in the isotonic method of recording muscular contractions. 

 Under the conditions of an ordinary physiological experiment, a contracted 

 muscle loaded only by a light lever is shorter than the non-contracted, 

 but can be stretched to the length of the latter by a certain weight, when 

 it will be in a condition of tension. In their natural position in the body 

 muscles may possess any length between extreme shortening and extreme 

 elongation whether they are in a resting or in an excited condition. Since 

 the relaxed muscle requires only a minimal force to extend it to the maximal 

 length possible in its natural relationships in the body, it is" usual to speak 

 of the different lengths of an excited and unexcited muscle, the lengths being 

 in this case those which are impressed on the muscle by a minimal load. 

 When we measure by means of the isometric method the maximum energy 

 set free in a muscle as the result of excitation, we find, as Blix first pointed 

 out, that this energy depends on the length of the muscle fibres during 

 the period of contractile stress set up by the excitation. With increase 

 in the length of the muscle the tension developed on excitation increases 

 until the length of the muscle is somewhat greater than that which it possesses 

 in its normal relationships in the body. To lengthen the muscle beyond 

 this point a certain stretching force must be applied to it which rapidly 

 increases. The tension developed on excitation however soon begins to 



iminish. 



These relationships are shown by the diagram (Fig. 61), where the ordinates repre- 

 snt the length of the muscle and the abscissae the tension on the muscle. The left- 

 id thick line represents the muscle in a state of rest, the right-hand curved line 

 the muscle in a state of excitation. The horizontal distance between the two lines 

 gives the increase of tension (as measured by the isometric method) produced when the 

 muscle passes from the resting into the excited state as the result of stimulation by a 

 single induction shock. 



t Since the tension set free on excitation depends on the length of the 





