452 MANUAL OF PHYSIOLOGY. 



CHANGES OCCURRING IN MUSCLE ON ITS ENTERING THE 

 ACTIVE STATE. 



Changes in Structure. The examination of muscle with the 

 microscope during its contraction is attended with considerable 

 difficulty, and in the higher animals has not led to satisfactory 

 results. In the muscles of insects, where the differentiation of 

 the contractile substance is more complicated, certain changes 

 can be observed. The fibres, and even the fibrillse within them, 

 can easily enough be seen to undergo changes in form correspond- 

 ing to those of the entire muscle, namely, increase in thickness 

 and diminution in length. A change in the position and relative 

 size of the singly and doubly refracting portions of the muscle 

 element has been described, and some authors state that the lat- 

 ter increases at the expense of the former after an intermediate 

 period in which the two substances seem fused together. 



Chemical Changes. During the contracted condition the chem- 

 ical changes which go on in passive muscle are intensified, and 

 certain new chemical decompositions arise, of which, however, 

 not much is known. 



Active muscle takes up more oxygen than muscle at rest, as is 

 shown by the facts that, during active muscular exercise, more 

 oxygen enters the body by respiration, and the blood leaving active 

 muscles is poorer in oxygen than when the same muscles are 

 passive. This absorption of oxygen cannot be detected in a 

 muscle cut out of the body, nor is any supply of oxygen neces- 

 sary for a contraction of such a muscle, since a frog's muscle will 

 contract in an atmosphere containing no oxygen. From this it 

 would appear that a certain ready store of oxygen must exist in 

 some chemical constituent of the muscle substance ; and it is pos- 

 sible that some chemical compound, which is constantly renewed 

 by the blood existing in the muscle, is its normal source of oxygen, 

 and not the oxyhsemoglobin of the blood. 



The amount of CO, given off by a muscle increases in its state 

 of activity, as may be seen by the greater elimination from the 

 lungs during active muscular exercise, and by the fact that the 



