PHYSIOLOGICAL 



237 



easily attaining to a velocity of a mile in a minute. The heron is no 

 laggard on flight, but it strikes with its wings no more than twice 

 in a second ; we have to think of a bee with more than two hundred 

 strokes in the same time ! 



The Contraction of Muscle. — Now pressing the question 

 farther home, we must ask what happens chemically or biochemically 

 when a muscle contracts. Modern advance in this field began with the 

 studies of Fletcher and Hopkins on the appearance and disappear- 

 ance of lactic acid in muscle cells, and the associated explanation 

 that the muscle fibre is a more or less permanent structure of 

 parallel threads of protein, which tend to contract strongly if their 

 surroundings become acid. It appears that the nerve-message to the 

 muscle, "ordering" it to contract, causes lactic acid to be formed 

 from some previous substance, and that this lactic acid by its mere 



Fig. 45. 



A Small Piece of Striped Muscle, showing the Fibrils. Note the alternation 

 of longer dark (3) and shorter light bands (2-4) ; and a median line (i) 

 across the middle of each light band. 



presence causes the muscle fibres to contract; before the muscle 

 can return to the normal again, the lactic acid has to be removed. 

 The studies of Meyerhof , Embden, and A. V. Hill have greatly added 

 to this part of the picture. 



Lactic acid is readily derived from sugar, of which there is always 

 a supply available in the muscle cell, which contains a store of the 

 complex carbohydrate glycogen. The sugar does not exist free, but 

 combined with phosphorus as hexose-phosphate ; but from this 

 combination lactic acid is easily liberated, and the fibre contracts. 

 This, it must be noted, is done without the intervention of oxygen 

 at all; the reactions are merely simplifications, not oxidations. The 

 problem of removing the lactic acid now arises, although some is 

 probably neutralised for the moment by the proteins present, and 

 here oxygen does play a part. But the whole of the lactic acid 



