CONTRACTILE TISSUES 447 



were driven and a store of air at high pressure obtained. The engine might then 

 be stopped and the compressed air used to drive pneumatic tools. This would be 

 similar to the anaerobic muscular contraction. Or the compressor might be kept 

 continuously at work, as is usual, in order to replace the potential energy of the 

 air consumed by the tools ; this corresponds to the work of a muscle under normal 

 oxygen supply. If it be preferred to regard the potential energy of the muscle 

 as chemical, then one might take the case of an engine driving a dynamo, which 

 is itself charging accumulators ; the current from the accumulators is then used 

 for electric motors. In either case, the energy of the fuel is not used directly, 

 just as that of the oxidation of carbohydrate in the recovery process of muscle is 

 not so used. The latter process is more efficient than the ordinary heat engine, 

 since the transformation of the chemical energy does not pass through the stage 

 of heat, although part of it appears to be lost in this way, even in muscle. 



It may be mentioned here that von Frey (1909, p. 497) clearly expresses the 

 view that only a part of the energy set free in active muscle is connected with 

 the contractile process itself, the other part being due to a subsidiary reaction, 

 for which oxygen is necessary. 



I venture to think that the phrase "oxidative removal" of lactic acid, although correct, 

 may be apt to give the mistaken impression that the lactic acid itself is oxidised. Perhaps 

 "restitution by another reaction" might be preferable, leaving the nature of the reaction 

 to be stated separately. 



Lactic acid, then, clearly undergoes no chemical change, and is merely put 

 backwards and forwards in some kind of physico-chemical system. It seems 

 remarkable that lactic acid, which is a common derivative of glucose, should 

 apparently have no connection with carbohydrate in the muscle. If one may say so, 

 it seems just calculated to put the physiologist on the wrong scent. It is no more 

 obvious why lactic acid particularly should have the properties necessary for the 

 act of contraction ; as a stage in the oxidation of glucose, it is at hand. In the 

 present state of knowledge as to the intimate nature of the process, any suggestions 

 must be purely speculative. But it seems probable that hydrogen ions, arising 

 from dissociation of the acid, play an important part in the polarisation of the 

 cell membranes, and also in the separation of inorganic salts from adsorption by 

 colloids in the sarcomeres, as in Macdonald's theory (1908), which is similar to that 

 already referred to in the case of nerve. In muscle, however, these electrolytes 

 which are set free owing to aggregation of colloids, are represented as increasing 

 the osmotic pressure of the contents, and causing shortening by attracting water 

 from one part of the fibre to another. 



It is an experimental fact that fatigued muscle has a higher osmotic pressure than resting 

 muscle, since it swells in a solution which is isotonic for the latter, and models have been 

 made which shorten when distended by forcing in water. Roaf (1914) has calculated that the 

 rate of inflow of water may be sufficiently great to offer no difficulty in this theory of contrac- 

 tion. It must also be admitted that, although the energy of a contraction is a function of the 

 area of certain surfaces in the fibre, the fact does not necessarily exclude the possibility of the 

 intervention of volume energy due to the osmotic pressure of the electrolytes split off from 

 these surfaces. 



The method described by Roaf (1913), by which electrodes of various types are used to 

 detect changes in the concentration of particular ions on the surface of muscle in contraction, 

 will probably afford valuable information, when complete, as to the time relations of the 

 muscle processes. This investigator has found an increase of hydrogen ions, a probable 

 increase of chlorine ions, and a diminution of oxygen tension in this way. The hydrogen 

 ions, no doubt, come from lactic acid and the chlorine ions from potassium chloride, which 

 might either be set free from adsorption or escape owing to changes of permeability. 



There is a further group of theories which attributes the development of 

 tension in a muscle to changes of surface tension at the contact of fibrillae with 

 sarcoplasm. That changes in surface tension are a controlling factor in the 

 development of the energy of muscular contraction is made practically certain by 

 the observation of Bernstein (1908), who found that the maximal tension developed 

 by a particular muscle, for example, was 375 g. at and 205 g. at 18. 

 This means that the energy in question has a negative temperature coefficient, and 

 of all the possible forms of energy involved in muscular processes, surface energy 

 is the only one that has a negative coefficient. This follows from the fact that the 

 surface tension at the interface between a liquid and its vapour becomes zero 



