WORK PRODUCTION 533 



katabolism of materials contained in the muscle or brought to 

 it by the circulation together with a corresponding transfor- 

 mation of their chemical energy. This katabolism is in effect 

 an oxidation, yielding chiefly carbon dioxid and water, but as 

 to the details of the process, the views of physiologists differ. 



Certain general features of muscular katabolism are fairly well 

 made out. First the immediate accompaniment of contraction is 

 not an oxidation but a rapid, almost explosive, breaking down of a 

 substance or substances present in the muscle, causing the produc- 

 tion of carbon dioxid. It has been shown, according to Zuntz and 

 Loewy, that the muscle contains no free oxygen. Nevertheless, it 

 contracts instantaneously when stimulated, while the effects upon 

 the blood supply follow later, circulation and respiration being stimu- 

 lated by the carbon dioxid and other products formed. Further- 

 more, it has been shown that, under certain conditions at least, a 

 muscle may continue to contract and give off carbon dioxid in the 

 entire absence of oxygen. 



With continued activity of the muscle, there is established more or 

 less distinctly a state of equilibrium with the increased blood supply, 

 oxygen being taken up by the muscle and carbon dioxid given off, 

 while, according to a number of experimenters, the dextrose of the 

 blood also disappears during its passage through the muscle. Other 

 products of muscular katabolism, notably lactic acid and potas- 

 sium mono-phosphate the so-called fatigue products tend to 

 accumulate in the muscle and diminish and finally suspend its ability 

 to respond to a stimulus. Fatigue of the muscles usually results 

 from a gradual accumulation of these substances and not from lack 

 of material to be katabolized. 



631. Energy transformations. The katabolism of matter 

 which takes place in muscular contraction implies an equiva- 

 lent conversion of chemical energy into kinetic energy. The 

 energy thus transformed appears finally in the two forms of heat 

 and visible motion (work) though the ratio between the two 

 may vary widely under different conditions. As regards the 

 intermediate stages of this process, relatively little certain 

 knowledge is yet available. Broadly, it may be said that 

 there are two possible general views. The first of these con- 

 siders that the potential energy of the material katabolized is 

 first converted into heat, and that subsequently a portion of 

 this heat is converted into mechanical motion. The second 



