THE INTIMATE NATUEE OF MUSCULAR CONTRACTION 235 



from an oval to a spherical shape. It would be impossible however for 

 any large changes of temperature to take place in the muscle without entirely 

 destroying its chemical character, and with small differences of tempera- 

 ture it would be impossible to attain the efficiency of 50 to 100 per cent, 

 which characterises muscle. 



Under certain conditions we may obtain by a machine almost the entire 

 energy of a chemical change. The condition is that the chemical change 

 shall be susceptible of taking place in a galvanic battery. We may use, 

 for instance, a series of Daniell cells to drive an electric motor and allow 

 the motor to perform mechanical work. Under these circumstances we 

 could theoretically obtain ICO per cent, of the total chemical energy avail- 

 able, and in conditions of practice the efficiency of the machine may attain 

 to 70 or 80 per cent. A similar arrangement might be present in the ultimate 

 contracting elements of the muscle fibre. The mechanism in the fibre must 

 be one which will provide for a more or less direct transformation of chemical 

 energy into mechanical energy without a previous conversion of the chemical 

 into heat energy. In the living body, where everything is in solution, all 

 the energies may be reduced to one of two kinds, osmotic energy and surface 

 energy. The contractile machine must therefore be one which employs 

 one or other, or both, of these forms of energy. We might with Macdougall, 

 regard the contractile element as a cylindrical structure differing in its 

 contents from the surrounding sarcoplasm. When the muscle is at rest 

 the contents of the muscle prism will be in equilibrium with the surrounding 

 sarcoplasm. We might imagine the excitatory process to consist in a sudden 

 chemical change occurring in the contents of the muscle prism. The 

 production of a number of new molecules within the muscle prism (e.g. of 

 lactic acid) would raise the osmotic pressure within the prism and occasion 

 a rapid flow of water from the sarcoplasm. As a result the pressure in the 

 muscle prism would rise and cause a bulging of its lateral wall and a shorten- 

 ing of the whole element. The subsequent phase of relaxation may be due 

 either to a secondary change, e.g. oxidation, leading to the formation of a 

 substance to which the walls of the prism are freely permeable, or to the 

 gradual leak of the primary products of oxidation or disintegration into the 

 sarcoplasm. The substance or substances giving rise to the osmotic differ- 

 ences which determine contraction may be either products such as lactic 

 acid and carbon dioxide, which are formed during contraction, or may 

 possibly be of the nature of neutral salts set free from some condition of 

 combination with the proteins of the sarcous element. Macdonald has 

 brought forward micro-chemical evidence of the appearance of potassium 

 salts in the sarcous element during the state of activity of the muscle. 



On the other hand, Bernstein has suggested that the changes during 

 muscular contraction are determined by alterations in surface tension. 

 If a little mercury be spilt on a plate the particles form globules. They are 

 kept from spreading themselves out in a thin film under the influence of 

 gravity in consequence of the surface tension of the mercury. Any modifica- 

 tion of the surface will alter the tension, and therefore state of expansion, of 



