52 THE HUMAN MECHANISM 



Let us not lose sight of the central fact. The activity of a 

 muscle fiber, like the activity of a gland cell, is the result of a 

 chemical change within the cell. In both cases the food mate- 

 rial derived from the blood is transformed into something else 

 and activity is accompanied by the production of new sub- 

 stances. In the case of the gland these new substances, or 

 part of them, go to form essential constituents of the secre- 

 tion, and we see at once the end secured by the chemical 

 change. In the case of the muscle the end is, at first sight, 

 not so clear. The substances formed are not of obvious use 

 to the body, and we have now to inquire how this chemical 

 change serves the purpose of producing a muscular contraction. 



11. Relation of the chemical changes to the work of muscular 

 contraction. It is a familiar fact that chemical changes often 

 yield power for work. The explosion of dynamite (a cleavage 

 change), for example, will shatter large masses of rock ; 

 the oxidation of coal in a locomotive engine supplies the 

 power to move a heavy train of cars. In both cases waste 

 products are produced, and in the change which produces 

 them power is liberated; but in order that this power may 

 be utilized to do work, some mechanism is needed to apply 

 it to the desired end. The burning of coal in an open grate 

 liberates power, but in the absence of any mechanism adapted 

 to that purpose, it does no work ; the same coal burned under 

 the boiler of an engine, with its mechanism of boiler, piston, 

 driving rod, and wheels, moves the train of cars. 



So it is with the muscle. Within the cytoplasm of the 

 fiber are the myofibrils (p. 35), and there are convincing 

 reasons for believing that the combination of myofibril and 

 sarcoplasm constitutes the mechanism of the muscle fiber. 

 The power liberated by the cleavage change acts upon this 

 mechanism, causing the shortening and thickening of the 

 myofibrils, whereby a pull is exerted on the tendon. Whether 

 the subsequent oxidative changes also contribute power for 

 the work or merely produce heat is still an open question. 



