OH. XIII. ] 



CHEMICAL CHANGES. 



155 



If two couples are in circuit, as shown in the second diagram, 

 and they are heated equally, no current will pass through the 

 galvanometer, the current through one couple being opposed by 

 the current through the other. But if the two couples are heated 

 unequally, the direction of swing of the galvanometer needle 

 indicates which is the wanner. To apply this to the frog's gastro- 

 cnemius, plunge several needle-shaped couples (diagram 3) into a 

 frog's gastrocnemius of one side and the same number of couples 

 into the gastrocnemius of the other side, and then excite first one 

 then the other sciatic nerve ; a deflection of the galvanometer will 

 be observed first in one, then in the other direction, indicating the 

 production of heat first on one side, then on the other. 



Chemical Changes in Muscles. 



The chemical changes which are normally occurring in a resting 

 muscle are much increased when it contracts. Waste products of 

 oxidation are discharged, and the most abundant of these is carbonic 

 acid. Sarco-lactic acid is also produced, and the alkaline reaction 

 of a normal muscle is replaced by an acid one. The muscles of 

 animals hunted to death are acid ; the acid reaction to litmus paper 

 of a frog's gastrocnemius can be readily shown after it has been 

 tetanised for 10 to 15 minutes. 



The quantity of oxygen consumed is increased, but the con- 

 sumption of oxygen will not account for the much greater increase 

 in the discharge of carbonic acid. This is illustrated by the 

 following table : 



Indeed, a muscle can be made to contract and give off oxidation 

 products like carbonic acid in an atmosphere containing no oxygen 

 at all. The oxygen used is thus stored up in the muscle pre- 

 viously. The oxygen is not, however, present in the free state, 

 for no oxygen can be detected in the gases obtained from muscles 

 by means of an air-pump. Hermann has supposed that the 

 oxygen enters into the formation of a complex hypothetical 

 compound he calls inogen. On contraction he considers this is 



