CONTRACTILE TISSUES 439 



no tension, oppose no resistance to stretching, and will not return to its original 

 length when released. When a muscle is stimulated, it changes its physical state 

 from that of the stretched coil of lead wire to that of the stretched coil of steel wire. 

 The fact may also be realised if we allow a muscle to shorten on stimulation and 

 then pull it back to its original length, the stimulation being continued. It will 

 require a certain force to do so ; this force may be measured by the weight which 

 it is necessary to hang on the end of a vertical muscle in order to bring it to its 

 resting length. This is, of course, only a rough measurement, because the weight 

 will passively stretch both the resting and the contracted muscle ; so that it will 

 be found that if the weight is applied which is just sufficient to extend the muscle 

 to its unloaded resting length, the weight will fall somewhat when the stimulus 

 ceases, since it stretches the resting muscle to a greater length than its unloaded 

 one. This is the fact involved in the principle of " after-load" in which the weight 

 is supported at the position of the length of the resting muscle and -does not 

 stretch it until contraction takes place. 



As we shall see in considering the heat evolved and also in the theory of 

 muscular contraction, the development of tension is the fundamental fact in the 

 process, so that it is of importance to grasp its meaning clearly. 



The special case of certain muscles which are able to possess the same degree 

 of tension at different lengths requires separate consideration and will be discussed 

 in Chapter XVIII., as it would tend to confuse the issue of the problem before 

 us here. 



FORMS OF CONTRACTION 



It is not my intention to give details of the varied phenomena to be observed, 

 especially in skeletal muscle, when stimulated in different ways or when the load 

 is applied or removed at different stages of the course of a contraction. There 

 are some which are necessary for our further discussion and details of the others 

 will be found in the textbooks of Human Physiology. The articles by von 

 Frey (1909) on striated muscle and by Griitzner (1904) on smooth muscle may be 

 consulted. 



When a single electric shock is applied to the nerve of a nerve-muscle prepara- 

 tion, nothing happens that can be seen for a period of two- or three-thousandths 

 of a second, as we have already learned. The muscle subsequently contracts at a 

 certain rate and relaxes again. It is not always remembered that I he processes 

 both of contraction and of relaxation are not instantaneous. The curve can be 

 traced, on moving smoked paper, by a lever to which the muscle is attached. The 

 rise is gradual and so is the return. But mere inspection of the curve does not 

 tell us whether the rate of fall was that of a body falling freely, or whether it was, 

 so to speak, allowed to fall gradually by a gradual disappearance of the state of 

 contraction. Tracings in which the lever is released at the top of contraction 

 show that the rate of its fall in such a case is greater than when connected with 

 the relaxing muscle, so that the state of contraction does not cease suddenly at 

 the top of the curve, but disappears gradually. At the same time, as we shall see 

 later, the active process of contractile" stress, or, in other words, the development 

 of energy, ceases at the top of the curve, so that changes of tension applied after 

 this point, do not affect the total amount of energy developed. 



When a muscle is held so that it cannot shorten, its contraction is said to be 

 isometric, since its length does not alter ; when it is allowed to shorten in such a 

 way that it raises a weight or stretches a spring, the weight being applied in a 

 manner such that its inertia does not come into play, or the spring such that its 

 tension remains constant, the contraction is isotonic, 



It will be obvious that a contraction may be of one type in a part of its course, 

 and of another type in the remainder. Since the tension develops gradually, a 

 muscle is able to raise a weight at a later period of its contraction, which it was 

 unable to raise at an earlier stage. Thus the contraction is first isometric, then 

 isotonic. On the other hand, the weight raised may suddenly come against an 

 unyielding obstacle ; the contraction is then first isotonic, then isometric. The 

 contraction of the heart muscle is first isometric, then, after the aortic valves have 



