MUSCLE CONTRACTION. 457 



MUSCLE CONTRACTION. 



Change inform. The most obvious change a muscle undergoes 

 in passing into the active state is its alteration in shape. It 

 becomes shorter and thicker. The actual amount of shorten- 

 ing varies according to circumstances, (a) A muscle on the stretch 

 when stimulated will shorten more in proportion than one whose 

 elasticity is not called into play before contraction, so that a 

 weighted muscle shortens more than an unweighted one with the 

 same stimulus. (6) The fresher and more irritable a muscle is, 

 the shorter it will become in response to a given stimulus; and, 

 conversely, a muscle which has been some time removed from the 

 body, or is fatigued by prolonged activity, will contract propor- 

 tionately less, (c) Within certain limits, the stronger the stimulus 

 applied the shorter a muscle will become, (d) 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 shorten- 

 ing during contraction, so that there is but little change in bulk. 

 It is said, however, to diminish slightly in volume, becoming 

 less than y-^^th 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 capil- 

 lary 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 den- 

 sity during contraction, the hardness which is communicated to 



