RAPIDITY OF MUSCULAR CONTRACTIONS. 247 



will contract on stimulation, arid lift the load from T to Q. In so doing it 

 will perform work represented by the area QRST. If, however, where 

 the muscle has contracted to Q, a fraction of the load be removed, it will lift 

 the rest and perform more work. If, for instance, 5 grins, be removed, it will 

 lift the remaining 35 grms. to X, and do an additional amount of work repre- 

 sented by the area XYEQ. If this be continued, it will finally contract to 0, 

 and perform its maximum work represented by the area OSTQ. This area 

 may be measured and compared with the parallelogram abed either by using 

 a planimeter, or by cutting out corresponding areas in a piece of cardboard 

 and weighing them, or by plotting out on squared paper. Many muscles in 

 the body encounter, during the course of their contraction, less and less 

 opposing force, and approximate in some degree to the ideal case represented 

 above. The reason for this will be understood when the movements around 

 the joints are studied, for then it will be seen that, according to the position 

 of the limbs, the force of muscular contraction is very frequently opposed to 

 a diminishing resistance (see p. 253). 



From the definition of the term " work," as given above, a peculiar diffi- 

 culty arises in discussing animal mechanics. It would seem from it that, 

 when a muscle remains contracted and supporting a weight, it does no 

 mechanical work while this is going on. " Work " being the product of a force 

 into a space, there can be no " work " done when no space is traversed. But 

 the muscle after a time becomes exhausted, and it is found to contain 

 products of chemical action. This shows conclusively that work is actually 

 being done, though it does not appear as external work, and cannot be gauged 

 by the ordinary mechanical standard. 



The rapidity of muscular contractions. -When a muscle contracts 

 it does so with a certain rapidity, shortening for a certain distance in a 

 certain interval of time. A survey of animal movements shows us that 

 these differ greatly in their velocity, from the slow movements of the 

 muscular walls of the stomach to the rapid movements of the insect's 

 wing. We find that there are, in nature at least, three ways by which 

 increased velocity of movement may be obtained. In the first case, the 

 muscular fibres themselves acquire the faculty of moving more quickly 

 and imparting a greater velocity to the hard tissue to which they are 

 attached. This change is associated with a marked structural alteration 

 they become striped. Thus the muscular tissue of molluscs in general 

 is unstriped ; but certain species, together with more active contraction, 

 have acquired striped muscular tissue. 



In the second case, the muscle is attached nearer to the fixed point 

 of a lever on which it acts. Thus, while the biceps, attached to the 

 radius near the elbow-joint, moves its point of attachment with but a 

 small velocity, the hand at some distance from the elbow moves 

 comparatively rapidly through space. 



But there is a third method, by which a greatly increased velocity 

 may be obtained, a method almost entirely overlooked by physiological 

 investigators, although it has not escaped the observation of A. Tick. 

 Helmholtz 1 has shown that the full force of muscular contraction 

 takes some time to develop. The muscle, as it were, gets up steam, 

 so that a movement is made, not at full steam pressure, but begins as 

 soon as the absolute muscular force just overbalances the resistance. It 

 follows from this that if the point of attachment of the muscle be fixed 

 in space, for, say, a second after the muscle has begun to contract (but 

 not to shorten), and if it be then let go, the muscle will have developed a 



1 Arch.f. Anat., Physiol. u. ivissensch. Med., 1850, S. 276 ; and 1852, S. 199. 



