264 PHYSIOLOGY 



the platinum coil. He imagined that the chemical changes in the 

 muscle liberate heat and that the effect of this heat upon the doubly 

 refractive particles is to make them imbibe the surrounding water 

 so that they change 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 temperature it would be impossible 

 to attain the efficiency of 12 to 20 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 100 per cent, of 

 the total chemical energy available, 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 every- 

 thing 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 may, 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 sarco- 

 plasm. We may imagine the excitatory process to consist in a sudden 

 chemical change, either of disintegration or of oxidation, occurring 

 in the contents of the muscle prism. As a result there is a production 

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

 or of carbon dioxide), which raises the osmotic pressure within the 

 prism and occasions a rapid inflow of water from the sarcoplasm. 

 As a result the pressure in the muscle prism rises and causes a bulging 

 of its lateral wall and a shortening of the whole element. The subse- 

 quent phase of relaxation may be due either to a secondary change 

 in the products of 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 

 differences 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 



