MUSCLE 267 



tract, moving the limbs, and the shortening is more extensive 

 than mere coagulation of muscle-plasm would account for. 

 It is evident that a process similar to functional contraction 

 precedes coagulation. Many a watcher in the chamber of 

 death has been startled by the shaking of the bed. Even a 

 sound resembling a sigh may be caused by contraction of the 

 muscles of the chest. Placing his hand over the region of 

 the heart, the attendant finds the body warmer than it was 

 when life became extinct, for much oxidation has since taken 

 place. 



What chemical changes occur in muscle when it contracts ? 

 What is the chemical source of its power ? Carbonic acid is 

 given off. This is the only product which we can collect and 

 measure ; but it is taken for granted that hydrogen atoms also 

 combine with oxygen, forming water. There is no reason for 

 thinking that nitrogen is removed from the molecules of its 

 protoplasm with any greater rapidity during the activity of 

 muscle than when it is quiescent (cf. p. 212). Is the oxida- 

 tion immediate and complete, or does it occur in stages ? 

 For many years attention has been directed to lactic acid, 

 partly because this substance is found in muscle which has 

 been made to contract under experimental conditions, partly 

 because, on theoretical grounds, glycogen (animal starch) is 

 looked upon as the most important of muscle-foods. Lactic 

 acid C 3 H 6 3 has the same percentage composition as 

 glycogen C 6 H 12 6 . Its formation from glycogen merely 

 involves a rearrangement of atoms. It has been supposed 

 that lactic acid is formed in the first instance, and then, if the 

 supply of oxygen be sufficient, oxidized to carbonic acid and 

 water. But this hypothesis may be resisted on various 

 grounds. Undoubtedly, lactic acid appears when oxygen is 

 deficient. Under all circumstances and in all tissues a certain 

 amount of it is formed. There are reasons for thinking that 

 it carries away the nitrogen which is wasted, as lactamide. 

 But it does not follow that under normal conditions, when 

 muscle is abundantly supplied with blood, lactic acid appears 

 in any greater quantity during activity than during rest. 

 The hypothesis is due to the misconception which we have 

 already endeavoured to correct. It is difficult to get away 

 from the steam-engine analogy. A steam-engine is made of 



