96 INTRODUCTION TO GENERAL PHYSIOLOGY 



is at present impossible to say how the energy produced by the 

 oxidation of lactic acid is stored in the muscle. 



Some light is thrown on the nature of the mechanism which 

 causes the characteristic state of tension by two experimental 

 facts : 



I. It is found that the magnitude of the tension developed, and 

 therefore of the work done, is greater the longer the fibres of the 

 muscle are at the moment when the state of tension is brought 

 about. If the muscle is stretched, a more vigorous contraction is 

 obtained. This applies, naturally, only within such limits as not 

 to affect the muscle fibres injuriously (E., p. 206). The fact shows, 

 in the first place, that change of volume of some elements of the 

 structure is not the determining factor in the process, because the 

 volume is not altered by stretching the muscle. In other words, 

 we cannot look for an explanation of the origin of the tension in 

 osmotic forces. What has been increased in the experiments 

 referred to is the length of the fibres and certain constituents in 

 them. This means that there has been an increase in' the area of 

 certain surfaces arranged longitudinally. We think at once of that 

 property of boundary surfaces that results in surface tension, and 

 that it is by changes in this surface tension that the state of tension 

 of the muscle, as a whole, is produced. Now, what does the 

 structure of muscle suggest? Examination of the microscopic 

 structure of that kind of muscle with which we are dealing, the 

 voluntary or skeletal muscle, which is under the control of the will 

 and moves parts of the bony skeleton, we find that it is composed 

 of long narrow fibres of a protoplasmic material ("sarcoplasm "), in 

 which are embedded a number of very fine threads of somewhat 

 complicated nature (" fibrillse ") (E., p. 206). We have provision, 

 therefore, for the boundary surface between phases demanded by 

 the surface tension theory. When hydrogen ions make their 

 appearance in consequence of the formation of lactic acid, or other 



( acid, at this contact surface between sarcoplasm and fibrillse, a 

 1 change in surface tension results. The surface energy provided 

 ^ by this is in proportion to the area of surface on the fibrillae or to 

 their length, as found by experiment. 



II. The second fact which tends to confirm the view that 

 surface forces are responsible for the tension of muscular contrac- 

 tion, is that the tension developed is higher at a low temperature 

 than at a higher one (E., p. 206). As we saw in an earlier chapter, 

 this negative temperature coefficient is a peculiarity of surface tension, 

 so far as concerns those various phenomena which could play a 

 part in the process. 



The fact itself is shown also by the behaviour of smooth muscle, 



