DISCOVERY 



65 



ture. There can be no doubt, then, that, whilst " rest- 

 ing " the isolated muscle we are talking about is under- 

 going the chemical change of oxidation, necessary, 

 so to speak, to keep the fires of life burning inside the 

 living cell. 



Death and Recovery 



If the muscle be deprived of oxygen it dies much 

 sooner, but it does not die at once. The rate at which 

 heat is produced falls to a certain amount and then 

 remains constant for many hours. During this phase 

 part of the substance glycogen stored within the 

 muscle is converted into the substance lactic acid, and 

 ultimately the muscle dies in rigor mortis, due to the 

 accumulation of lactic acid. If, however, before 

 dying it be supplied again with oxj'gen, the rate at 

 which heat is produced rises not merely to its original 

 value, but to a higher one, the excess of heat evolu- 

 tion making up for the deficit during the earlier lack 

 of oxygen. The muscle in " recovering" uses not onl}' 

 its former income of oxygen, but also the amount 

 that it would normally have taken during the time 

 it was prevented. During the recovery also the 

 accumulated lactic acid disappears and the glycogen 

 from which it arose is largely restored within the 

 muscle cells. 



In the resting muscle, then, it is clear that two 

 processes connected with life are occurring: (i) a 

 process not requiring oxygen which leads to the 

 production of lactic acid from the carbohj'drate 

 glycogen ; and (2) a process requiring oxygen which 

 leads to the disappearance of the lactic acid and to 

 the restoration of the carbohydrate. In resting life, 

 in presence of sufficient oxygen, these two processes 

 appear superficially to be one, but, by first depriving 

 the muscle of the necessary oxygen and later restoring 

 it, it can be shown that the normal process of oxidation 

 occurs in two stages, the initial or " anaerobic " phase 

 and the recovery or " oxidative " phase. The first 

 of these "sets the pace" for the whole process; 

 the recovery phase merelj' follows and completes the 

 initial breakdown. 



The Function of Oxygen 



By writing down chemical formulEE it is not difficult 

 to show how the quantity of carbon dio.xide produced 

 by a muscle in presence of an adequate supply of 

 o.xj-gen corresponds e.xactly to the oxygen used, and 

 equally whether it be lactic acid or the carbohydrate 

 that is oxidised. These facts may be taken for 

 granted. In the muscle without oxygen, lactic acid, 

 as has been said, is produced from glycogen, the 

 glycogen changing into no other chemical compound. 

 This change is accompanied bj^ a production of heat. 

 In the recovery phase, however, although all the 

 lactic acid vanishes and glycogen appears again, the 



change is not a complete transformation of one into 

 the other, for a portion of one or of the other is oxidised 

 to supply the energy lost as heat in the earlier phase. 

 It is usual to suppose that it is a portion of the 

 lactic acid which is oxidised for this purpose, and if 

 this is so, we calculate that five-sixths of the lactic acid 

 is changed into glycogen and the remainder oxidised. 



The same clear division of the breakdown process 

 into two separate phases can be detected in a muscle 

 which is stimulated. An isolated muscle at rest in 

 oxygen contains very little lactic acid ; the more 

 accurately the estimation be made, the less appears 

 to be the lactic acid present. If, however, it be 

 stimulated electrically, heat is produced, and relatively 

 large quantities of lactic acid are formed. If the 

 stimulation be continued, it is found that a maximum 

 amount is eventually formed which is about 0-25 to 0-4 

 per cent, of the muscle's weight, depending upon con- 

 ditions. If the muscle can be kept alkaline, for 

 example, more lactic acid will be produced than other- 

 wise. If now such a muscle be left in o.xygen, the 

 lactic acid disappears, oxygen is used, heat and carbon 

 dioxide are formed ; one-sixth of the lactic acid being 

 oxidised, the remainder transformed into glycogen. 

 This is exactly as in the muscle " recovering " at rest 

 from oxygen deprivation. Moreover, other tests have 

 shown that the chemical change in the first phase is 

 totally uninfluenced by the presence of oxygen in 

 any way ; oxygen is used, but entirely in the second 

 phase, the recovery. Here is, then, one of the most 

 important generalisations of chemical and physical 

 biology, for it is probably as true of every kind of 

 tissue as of muscle : Oxygen is used by the living cell 

 only in what, properly speaking, may he called recovery 

 processes. 



Credit and Security 



The occurrence of oxidation is a necessary accom- 

 paniment of all the higher forms of life : continued 

 activity is entirely dependent on an adequate supply 

 of oxygen. To take an analogy from economics : 

 business activity is always accompanied by the 

 surrender of one's own goods in exchange for those of 

 someone else : continued activity is entirely dependent 

 on an adequate production of goods suitable for 

 exchange. In most businesses, however, occasions 

 of special need or opportunity occur, when it is neces- 

 sary to incur Habilities greater than can be covered 

 by the contemporaneous production of exchangeable 

 goods. Here, on the " security" of other assets, not 

 so readily exchangeable, a " credit " is obtained to 

 meet the special need. So it is in the body. If the 

 organs of the body were capable only of exerting 

 activity covered by their simultaneous supply of 

 oxygen, then that activity would be very limited, no 

 great effort could be made, no exceptional need met 



