138 



THE MUSCLE CELLS 



of surface tension at the interface between fibril and sarcoplasm 

 will cause the fibril to contract to its fullest extent. Of course 

 the student will understand that the subject bristles with diffi- 

 culties, and the above explanation can only be taken as a very 

 crude and incomplete basis for further elaboration. 



The only metabolite of muscular contraction that stands on a 

 very firm experimental foundation is lactic acid. In the absence 

 of oxygen muscle becomes acid. If contraction takes place in 

 the presence of oxygen, lactic acid does not appear. Lactic acid 

 formation in muscle has been a veritable red herring drawn 

 across the track. The problem looks so simple with lactic acid 

 as an intermediate step in the oxidation of carbohydrates. The 

 careful investigations of Prof. A. V. Hill of Manchester have made 

 it clear that no such simple process is possible. First, the lactic 

 acid is not a step in the oxidation of carbohydrates, and secondly 

 it is not oxidised itself to CO 2 and H 2 O as was thought. It is 

 not part of the fuel but is a necessary part of the machine, moving 

 to and fro like the piston of a gas engine. The arrival of the 

 stimulus (free kinetic energy) is the trigger setting free lactic acid 

 from some physico-chemical combination and so causing an 

 increase in H ion concentration and consequently an increase in 

 tension at the surface of the fibril and sarcoplasm. If this tension 

 is not allowed to cause contraction, the energy of tension can be 

 converted, as we have seen, into heat. The amount of heat so 

 obtained has been shown by Hill to be quantitatively equal to 

 the amount of work derivable from the tension. That is, the 

 efficiency of contraction is 100 per cent. 



(Mean maximum value = 91 per cent.) 



(2) Restitution. This extraordinarily efficient first stage of 

 muscular activity where stored energy is converted into work 



TABLE XXII. 



GASEOUS EXCHANGE IN M. Levator Labii Super ioris OF THE HORSE PER 



GRAM OF MUSCLE PER MINUTE (ClTAUVEAU AND KAUFMANN). 



