CONTRACTILE TISSUES 441 



or the electro-motive force of a concentration battery. Yet it would seem absurd 

 to draw the conclusion that the rate of discharge of a nerve cell has no connection 

 with chemical processes. Similar facts in the case of the rate of the mammalian 

 heart beat and other processes have been already referred to (page 43). A further 

 interesting fact found by Piper in the experiments quoted is that, at 37, the 

 nerve cells of the tortoise have the same oscillation period as those of the warm- 

 blooded vertebrate, namely, 47 to 58 per second. 



THE NATURE OF THE CONTRACTILE PROCESS 



It will perhaps facilitate comprehension of the relationship between the various 

 experimental facts, if we first of all consider a condensed statement of the view of 

 the processes taking place in active muscle, which the work of Hermann and others 

 in the past, but chiefly that of Fletcher, Hopkins, and A. V. Hill in recent years, 

 has made it necessary to adopt. 



When a muscle contracts, tension is developed and external work is done if the 

 tension is made use of to raise a weight or perform other functions requiring 

 expenditure of energy. It is obvious, therefore, that there must be something in 

 resting muscle which possesses potential energy of some kind, and that, on 

 excitation, some change takes place in this system resulting in loss of potential 

 energy. We know that lactic acid is formed and that the actual contractile 

 process is not associated with the giving off of carbon dioxide nor with the con- 

 sumption of oxygen. It is not, in fact, an oxidation, so that the " biogen " 

 conception fails here. Although there must be some large molecules, or aggregates, 

 containing the lactic acid group, these cannot be of a protein nature with " intra- 

 molecular " oxygen as one side chain and an oxidisable group at another place. 

 It appears that the potential energy must be in the form of surface energy or 

 osmotic energy, or both ; at all events, in some form which is not associated with 

 chemical reaction in the strict sense. At the end of the contraction, the cell 

 machinery possesses less potential energy and the systems actually participating in 

 the change, "inogens," if we may use Hermann's name, though not exactly in his 

 sense, have let loose lactic acid. 



Now to restore the system to its original state, with increase of energy content, 

 the lactic acid is put back by another reaction. In this process, the system is 

 restored to its original state of high potential energy, so that the reaction by 

 which it is effected must be one in which a considerable amount of energy is set 

 free. This is shown by the large consumption of oxygen and liberation of carbon 

 dioxide, indicating oxidation of some combustible substance. We have seen 

 already (page 271) that no nitrogen metabolism is associated with muscular work 

 as such ; the oxidised substance must therefore be carbohydrate or fat. It appears 

 that carbohydrate is actually used, but fat appears also to be capable of serving 

 the purpose, perhaps indirectly. 



After this condensed and somewhat dogmatic exposition, we may proceed to 

 consider the evidence on which the various statements are made. 



The production of tension, without shortening, is measured by the various 

 methods of tracing isometric curves, as mentioned above. The principle on which 

 these methods rest is that of arranging the muscle so that it shall pull against 

 a strong spring or twist a stiff wire ; thus the very slightest change in its own 

 length is sufficient to produce considerable tension in the spring. This very slight 

 movement is magnified by a long lever, or better by a reflected beam of light, 

 whose movement is recorded on the surface of a moving photographic plate 

 (" optical lever"). 



It is unnecessary to describe any particular experiment to show that work can 

 be done by a muscle allowed to shorten. Everyday experience in the raising of 

 weights is sufficient to prove this point. 



That this work is done at the expense of potential energy stored in the muscle, 

 and not by an exothermic chemical reaction involving the burning up of some 

 food-stuft' at the moment, is shown by the fact that an excised frog's muscle in 

 nitrogen is capable of giving a maximal contraction every five minutes for two 



