244 



ANIMAL MECHANICS. 



ordinates the shortening which follows upon the application of a 

 uniform stimulus. Let us consider the position of the muscle at the 

 three points a, b, and c. At a the muscle is loaded with 10 grms., 

 and it is raised to a considerable height; the muscle is short. Its 

 strain balances the stress of the 10 grms., and more than 10 grins, would 

 elongate it. At I it is loaded with 20 grms., and at c with 30 grms. In 

 each case there is equilibrium between the muscular force and that of 

 gravity, and we notice that this muscular force is smaller when the 

 muscle is shorter. 



A simple method of demonstrating this fact is the following. 

 Stand upright in front of a table, bend one knee backwards so that the 

 toes touch the ground. Load the foot as by the weight of a child 

 sitting in front of the heel. If the child is of the right weight, you can 

 just lift it by trying to bend the knee still more. Now, bend the 

 body forward on the table so as to flex the thigh upon the trunk, and 

 the child can be raised with ease. In the latter case the flexor muscles 

 of the knee are nearly at their full physiological length, and their 

 absolute force is greater. 



The amount of shortening during contraction. When the muscle 

 contracts under the action of a given stimulus, it will shorten and 

 raise a weight to a height that will bear a certain ratio to its length. 

 Two similar muscles fastened tandem-wise to a weight will, on contract- 

 ing, raise it to twice that height. This distance will bear the same 

 ratio to their combined lengths, and with the same results these two 

 muscles might be replaced by one muscle of the length of the two 

 together ; and we may say, therefore, that the shortening of a muscle is 

 proportional to its length. This accounts for the fact that the extensive 

 movements of the fingers are not brought about by muscles situated 

 in the hand, but are produced by longer muscles lying in the forearm. 



By the same muscle, and with the same stimulus or effort, the lift 

 diminishes as the load increases. 



In Fig. 130 the abscissae represent the weights attached, and the 

 ordinates, lengths of muscles. The lower curve represents a muscle at 

 rest when loaded with 0, 10, 20, etc., grms., and the upper curve repre- 

 sents the positions assumed by the same point of the tetanised muscle when 

 loaded in a similar manner. It will be observed that the lift (aa") of the 

 unloaded muscle is greater than when the muscle is loaded with 

 10 grms. (&&'), and so on. Weber believed that the curves ultimately 

 cross, as represented in the figure. According to this observer, on 

 loading the muscle, with say 50 grms., and then stimulating it, it 

 lengthens instead of shortening. This is denied by more recent 

 observers, such as Fick. 1 



While Weber described a muscle as shortening 70 per cent, of 

 its length, when unweighted, more recent observers incline to put the 

 shortening at 20 to 30 per cent, of its length. 



Shortening of individual muscles. In order to investigate upon 

 the skeleton the possible shortening of an individual muscle, during a 

 movement of a limb, the following plan may be adopted. A hook is 

 driven into the exact place of insertion of the muscle to be investigated. 

 A staple is driven into the position of origin of the same muscle. A 

 piece of twine attached to the hook is passed through the staple and 

 tied to a weight, by means of which it is kept taut. The limb is placed 



1 "Untersuch. ueber Muskelarbeit, " Basel, 1867, 



