The Respiratory Process in Muscle. 



445 



Oxford, not to realise that in the study of muscle lay probably the first 

 path to knowledge of the inner processes of life within the living substance 

 itself. 



Intramolecular Oxygen and the Theory of " Inogen." 



The closing years of the nineteenth century, and with them the last 

 occasion on which the Croonian Lecturer dealt with the subject prescribed 

 hj the endowment, marked what seemed the final establishment of a 

 particular conception of respiration in muscle. It was almost universally 

 held that rmiscular energy and, by inference, the energy liberated in any 

 cell upon activity, whether as mechanical energy or heat, sprang from a 

 more or less explosive splitting of a molecular complex which had been 

 made highly unstable, that is " irritable," by the previous inclusion within it 

 of oxygen taken in by the cell during rest. The breakdown of this hypo- 

 thetical molecule was supposed to yield both lactic acid and carbon 

 dioxide, these being the two obvious and recognisable products of activity. 



The earliest phases of this conception, through all the stages of the long- 

 delayed discovery of oxygen, connected the idea of " irritability " directly 

 with that of combustibility. This notion, however, was negatived by the 

 discovery of Spallanzani that living tissues could long survive and continue 

 to yield carbon dioxide without any supply of oxygen except such as had 

 previously been available. This yield of carbon dioxide in the contemporary 

 absence of oxygen was shown to be true for the case of isolated muscle by 

 Muller, Liebig, Matteuci, and others, and it was upon this observation that 

 Hermann chiefly based his theory of inogen. He showed in 1867 that free 

 oxygen was not present in the air pumped from isolated frog's muscle, and 

 yet he found that without any fresh oxygen supply from outside, carbon 

 dioxide was yielded by the muscle when it contracted or when it stiffened 

 after death. At the same time, lactic acid was produced, as had previously 

 been shown, while no nitrogen bodies could be recognised as appearing. 



Hermann's hypothetical " inogen " accordingly was the unstable precursor 

 of both lactic acid and carbon dioxide, a precursor in which oxygen was 

 already cumbined, or placed in a position to combine, with carbon and 

 hydrogen in the combustion which was to yield the energy of contraction. 

 After the explosive breakdown of this precursor, it was supposed that fresh 

 carbon bodies, and perhaps also the lactic acid, were combined again in a 

 newly oxygenated unstable molecule of inogen. 



This inogen hypothesis of Hermann was taken up again ten years later by 

 Pfiiiger in his well known studies of " physiological combustion." .Here they 

 were amplified and illustrated with great wealth of rhetoric, but without 

 significant change and without fresh experimental support. Pfluger's 



