36 SCIENCE PROGRESS 



in the urine. The amount increases with the activity of the 

 muscles, the maximum being observed when strenuous mus- 

 cular work is done under conditions which interfere with 

 normal aeration through the lungs. Exertion at high altitudes, 

 where the oxygen tension of the atmosphere is low, has been 

 shown, for instance, to lead to an increase of lactic acid in the 

 blood. More precise information with regard to its signifi- 

 cance has been obtained by studying the processes which 

 occur in excised but still surviving muscles, especially in the 

 organs of cold-blooded animals, such as the frog, in which 

 chemical changes are slow and more easily analysed. In these, 

 the formation of lactic acid has been shown to be related 

 to the processes of surviving life. It ceases at a time when 

 the muscles no longer contract upon stimulation, so that the 

 production of the acid cannot be classed with post-mortem 

 changes. If the quiescent muscles are well supplied with 

 oxygen, lactic acid at no time appears in them in appre- 

 ciable quantity ; but if oxygen be available it accumulates 

 steadily up to the point of death. Now an excised muscle 

 can contract vigorously during a considerable period in the 

 complete absence of an oxygen supply. What then is the 

 source of energy under these conditions ? Since carbonic 

 acid is given off in the absence of a contemporary oxygen 

 supply, a belief, shared by Pavy, that the living tissues contain 

 " intramolecular oxygen " has long been held. Oxygen, it is 

 thought, is " built up " into the bioplasmic complex along with 

 oxidisable material. When energy is to be liberated there is 

 a change within the complex from less stable to more stable 

 configurations and oxidation products, especially carbon dioxide, 

 are produced. Recent critical experimental work has, in my 

 opinion, deprived this belief in intramolecular oxygen of all 

 foundation and I believe that its disappearance will mark an 

 advance in our understanding of living processes. The carbon 

 dioxide given off by a tissue when deprived of oxygen is 

 liberated from the alkaline carbonates, always present in tissues, 

 as the result of the accumulation of organic acids, of which 

 lactic acid is certainly the chief. Such carbon dioxide therefore 

 has no direct metabolic significance whatever. 



A very instructive observation has shown that when a 

 muscle is made to pass from a quiescent condition to one of 

 active contraction in the absence of oxygen, there is no increase 



