THE WORK OF ORGANS AND CELLS 51 



chemical element, .except that water is often added to the 

 material changed. In this process the combination of differ- 

 ent atoms which makes the compound is broken and the 

 molecule is split into two or more molecules. 1 Thus new 

 compounds or substances are formed. 



In the muscle fiber both these changes occur during con- 

 traction. Many, perhaps the majority of physiologists, now 

 think that the stimulus to the muscle fiber (nerve impulse, 

 electric shock) first causes a cleavage of the unstable fuel 

 of the fiber into lactic acid and possibly other products and 

 that this cleavage is the cause of the contraction ; under 

 normal conditions this is followed by an oxidation of the 

 lactic acid to carbon dioxide and water 



C 3 H 6 O 3 + 3 O 2 = 3 CO 2 + 3 H 2 O. 



On this view the cleavage takes place very suddenly (per- 

 haps requiring less than the hundredth of a second), while 

 the oxidation which follows requires several seconds or even 

 minutes for completion ; indeed, before it is complete, some 

 of the lactic acid may have passed out of the muscle fiber 

 into the lymph and blood. Some of the facts supporting this 

 view are the following: the lactic acid produced within the 

 muscle during contraction increases with the intensity of the 

 work ; the amount of it found after contraction is greater 

 when the supply of oxygen from the blood is diminished or 

 cut off ; and, finally, lactic acid disappears from the muscle 

 more rapidly after contraction, when the blood is well sup- 

 plied with oxygen, than when it is deficient in that element. 



1 Matter is composed of atoms of chemical elements ; these atoms are 

 combined or bound together to form molecules. A lump of sugar, for ex- 

 ample, would be composed of an inconceivable number of molecules of sugar, 

 and each molecule would consist of six atoms of carbon, twelve atoms of 

 hydrogen, and six atoms of oxygen bound together in chemical combination. 

 Sugar may undergo cleavage into lactic acid by splitting its molecule of 

 twenty-four atoms into two molecules of twelve atoms. The chemist ex- 

 presses this by the following equation : 



