114 THE BIOLOGY OF BIRDS 



In normal circumstances striped muscle contracts only 

 when impulses come to it from the central nervous system ; 

 but smooth muscle has an automatic activity, being able to 

 contract, apart from the central nervous system, under the 

 influence of local stimuli. The term " involuntary'," often 

 appHed to smooth or unstriped muscle, is apt to suggest 

 that it is not under the control of the central nervous system, 

 which is far from being the case. 



When a muscle contracts, e.g. in bringing the bird's 

 wing down, two very distinct processes occur. There is 

 potential energy in the resting muscle, in the form of surface 

 energy or osmotic energy, or both, and this is used to 

 develop tension which does work. In this physical process, 

 which might be compared to the uncoiling of a spring, 

 lactic acid is let loose from the muscle-substance. As 

 Professor Bayliss says : " If we compare the muscle to a gas 

 engine, the lactic acid corresponds to some essential moving 

 part, say the piston, which merely undergoes change of 

 position. Its change of position, however, leads to the 

 liberation of energy " (191 5, p. 446). 



The second chapter is putting the lactic acid back into 

 the muscle so that it may contract again. This requires 

 the oxidation of a carbohydrate {e.g. blood sugar) or, it 

 may be, fat, which supplies the energy for the restoration 

 of the lactic acid. Much oxgyen is used up in this combus- 

 tion, carbon dioxide is given oft", and heat is evolved. This 

 is the chemical phase of the process of contraction, succeeding 

 the physical phase. It appears that practically the whole 

 of the tension can be used to do work, but that about 50 per 

 cent, of the energy of the second phase is lost as heat. 



The important general fact, established by Fletcher 

 and Hopkins, is that the contraction of a muscle must be 

 separated into two chapters — a predominantly physical 

 chapter, without combustion, but with a displacement of 

 lactic acid, and a predominantly chemical chapter, centred 

 in an explosive combustion, as the outcome of which lactic 

 acid is replaced. 



To quote again from Professor Bayliss : " The muscular 



