66 



DISCOVERY 



or opportunity seized. Fortunately Nature has ar- 

 ranged that very violent efforts can be made, that 

 the body need not be content with its oxygen " income," 

 but that it can obtain a " credit " for future oxygen, 

 can (so to speak) " run into debt " for oxygen, b\' the 

 agency of the lactic acid which it releases as a " se- 

 curit J' ' ' for future oxidation. The extent of this 

 " credit " depends upon the lactic-acid-maximum, 

 and that depends upon the content of available alkali 

 of the muscle tissue. 



We have arrived then at two important generalisa- 

 tions, which appear to be established beyond dispute : 

 (a) that oxidation follows, and does not accompany 

 or precede, muscular activity ; and {b) that the 

 muscle, if its oxygen " income " be not adequate to 

 its momentary need, can obtain a considerable " credit " 

 for oxygen to be taken in later, on the " security" of 

 the lactic acid which it liberates within itself. 



Muscle as a Machine 



As to the function of this lactic acid, we are on more 

 speculative ground. It appears certain, at any rate, 

 that the acid provides part of the mechanism of the 

 machine, as well as the fuel to supply it with energy. 

 In a steam engine the coal is merely there as fuel ; 

 the steam is, so to speak, part of the mechanism itself, 

 it moves in and out in the cylinder and through the 

 valves, it applies a force to the piston. In an internal 

 combustion engine the gas is the fuel, its combustion 

 provides the energy ; it is, however, part also of the 

 mechanism, it moves in and out and applies a force 

 to the piston. In that sense the muscle resembles the 

 internal combustion rather than the steam engine ; 

 its fuel — lactic acid — is also part of the machinery. In 

 the internal combustion engine, however, all, or nearly 

 all, the gas admitted to the cylinder is burnt ; in the 

 muscle, only one-sixth of the lactic acid is oxidised, 

 the rest is restored to its original state. For another 

 reason, however, the analogy cannot be pressed. 

 For, although in the mechanical engineering sense 

 the muscle is as efficient as the best gas engine ever 

 made, it is not a heat engine. A heat engine needs a 

 difference of temperature through which to work ; a 

 muscle is throughout at uniform temperature. The 

 muscle acts indeed because it changes chemical energy 

 directly into icork at uniform temperature. In action 

 it is really more like the combination of two dry cells 

 and an accumulator. In lighting a lamp, or in any 

 other work for which it is used, an accumulator runs 

 down, the lead and the lead peroxide in it being 

 changed into lead sulphate; in "recovery" the 

 chemical energy stored in the dry cells charges the 

 accumulator by restoring the lead and lead peroxide 

 from the lead sulphate. But this analogy, too, is 

 incomplete. 



To reconstruct even in outline a physical picture of 

 the processes which occur in muscle leads to many de- 

 tails of a highly technical nature. It would seem 

 probable, however, that rrtany of the events occurring 

 in a muscle in action or at rest, can (or will shortly) 

 receive a reasonable and consistent explanation, in 

 terms of the processes of physical chemistry. 



We have spoken hitherto of the isolated muscle, at 

 rest or in activity. The same principles, however, 

 apply to the case of a man taking exercise. It is not 

 possible to record, without lag, the heat-production 

 of a living human muscle ; nor is it practicable to 

 determine the lactic acid present in it. By suitable 

 means, however, consisting of a mouthpiece, a pipe 

 and a bag, it is easy to allow a man to inhale ordinary 

 air of known composition and to collect all his expired 

 air in the bag ; then a measurement of its volume, and 

 an analysis of its contents, wiU enable us to calculate 

 the consumption of oxygen during even the most 

 strenuous exercise. Many interesting results can be 

 obtained with this method, which, although in practice 

 somewhat laborious, is in principle simple and reliable. 

 We will consider only a few of these. 



Oxygen Consumed in Exercise 



From the moment a man begins to make a violent 

 effort his respiration is rapidly and enormously in- 

 creased. At rest in bed the total air breathed per 

 minute by a man of 70 kilos (11 stone) body-weight 

 may be only about 6 litres (about lA gallons) ; running 

 at 12 miles per hour it rapidly rises to 120 litres or 

 more. This increase is not caused primarily by lack 

 of oxygen, but rather [a) by the increased production 

 of carbonic and lactic acids acting on the so-called 

 " respiratory centre " in the nervous system, and [b) 

 by other less tangible nervous influences due either to 

 consciousness of the effort or to reflex action caused 

 by it. The rate of oxygen consumption, however, 

 rises rapidly if the exercise be continued. Starting 

 at (say) 200 c.c. per minute when at rest, it rises to 

 a certain maximum value, characteristic of the violence 

 of the e.xercise, attaining this maximum to all intents 

 and purposes within two or three minutes. The more 

 severe the e.xercise, the greater is the final maximum 

 rate of oxygen-consumption associated with it; beyond 

 a certain limit, however, the mechanism for supplying 

 the oxygen fails ; in other words, the lungs, blood, 

 heart, and arteries have attained their maximum 

 activity, and no greater supply of o.xygen can be 

 secured, however much it be required. Consequently, 

 in all forms of violent exercise, after a certain limit 

 the body has to "go into debt " for the oxygen it 

 needs, over and above the amount it can obtain at 

 the time through the usual channels. This oxygen 

 it takes in later, after the exercise has ceased 



