460 MANUAL OF PHYSIOLOGY. 



applied, the shorter a muscle will become, (rf) A warm tempera- 

 ture augments the amount of shortening, the amount of contrac- 

 tion of frogs' muscles increasing up to 33 C. A perfectly active 

 frog's muscle shortens to about half its normal length. If much 

 stretched and stimulated with a strong current, it may contract 

 nearly to one -fourth of its length when extended. Muscles are 

 seldom made up of perfectly parallel fibres, the direction and 

 arrangement varying much in different muscles. The more 

 parallel to the long axis of the muscle the fibres run, the more 

 will the given muscle be able to shorten in proportion to its 

 length. 



The thickness of a muscle increases in proportion to its short- 

 ening during contraction, so that there is but little change in 

 bulk. It is said, however, to diminish slightly in volume, be- 

 coming less than y^U" smaller. This can be shown by making 

 a muscle contract in a bottle filled with weak salt solution so as 

 to exclude all air and to communicate with the atmosphere only 

 by a capillary tube into which the salt solution rises. The 

 slightest decrease in bulk is then shown by the fall of the thin 

 column of fluid in the tube. 



Since a muscle loses in elastic force and gains but little in 

 density during contraction, the hardness which is communicated 

 to the touch depends on the difference of tension of the semifluid 

 contractile substance within the muscle sheath. 



THE GRAPHIC METHOD OF RECORDING MUSCLE CONTRACTION. 

 In order to study the details of the contraction of muscle, the 

 graphic method of recording the motion is applied. The curve 

 may be drawn on an ordinary cylinder moving sufficiently rapidly. 

 Where accurate time measurements are required, it is better to 

 use one of the many special forms of instruments, called myographs, 

 made for the purpose. The principle of all these instruments is 

 the same, namely, an electric current, which passed through the 

 nerve of a frog's muscle connected with the marking lever, is 

 broken by some mechanism, while the surface is in motion ; the 

 exact moment of breaking the contact can be accurately marked 

 off on the recording surface by the lever which draws the muscle 



