430 CORRELATION OF PHYSICAL AND VITAL FOECES. 



vation of another portion to the state of living organized 

 tissue. 



The conditions of animal existence, moreover, involve a 

 constant expenditure of motor force through the instrumental- 

 ity of the nervo-muscular apparatus ; and the exercise of the 

 purely psychical powers, through the instrumentality of the 

 brain, constitutes a further expenditure of force, even when 

 no bodily exertion is made as its result. We have now to 

 consider the conditions under which these forces are devel- 

 oped, and the sources from which they are derived. 



The doctrine at present commonly received among physi- 

 ologists upon these points may be stated as follows : The 

 functional activity of the nervous and muscular apparatuses 

 involves, as its necessary condition, the disintegration of their 

 tissues ; the -components of which, uniting with the oxygen 

 of the blood, enter into new and simpler combinations, which 

 are ultimately eliminated from the body by the excretory oper- 

 ations. In such a retrograde metamorphosis of tissue, we 

 have two sources of the liberation of force : first, its descent 

 from the condition of living, to that of dead matter, involving 

 a liberation of that force which was originally concerned in 

 its organization ;* and second, the further descent of its 

 complex organic components to the lower plane of simple 

 binary compounds. If we trace back these forces to their, 

 proximate source, we find both of them in the food at the 



* It was by Liebig ("Animal Chemistry," 1842) that the doctrine was 

 first distinctly promulgated which had been already more vaguely affirmed 

 by various Physiologists, that every production of motion by an Animal 

 involves a proportional disintegration of muscular substance. But he seems 

 to have regarded the motor force produced as the expression only of the 

 vital force by which the tissue was previously animated ; and to have looked 

 upon its disintegration by oxygenation as simply a consequence of its death. 

 The doctrine of the " Correlation of Forces " being at that time undevel- 

 oped, he was not prepared to recognize a source of motor power in the ulte- 

 rior chemical changes which the substance of the muscle undergoes ; but 

 seems to have regarded them as only concerned in the production of heat. 



