PHYSIOLOGY OF MUSCLE TISSUE. 57 



disappear, giving to the fiber an appearance of homogeneity. There 

 is, however, no change in refractive power, as shown by the 

 polarizing apparatus. After the contraction wave has reached the 

 stage of greatest intensity, there is a reversal of the foregoing 

 phenomena, and the fiber returns to its original condition, which 

 is one of relaxation. 



Physical Changes. The extensibility of muscle is increased during 

 the contraction, the same weight elongating the fibers to a greater 

 extent than during rest. The elasticity, or its power of returning to 

 its original form is correspondingly diminished. 



Chemic Changes. The metabolism of muscle during the contraction 

 is very active. There is an increase in the production of carbon 

 dioxid and in the absorption of oxygen. The muscle changes from 

 an alkaline or neutral to an acid reaction, from the development 

 of sarcolactic acid. The muscle also becomes warmer. The electric 

 changes will be treated of in connection with nerves. 



Transmission of the Contraction Wave. Normally, when a 

 muscle is stimulated by the nerve impulse, the shortening and 

 thickening of the fibers begin at the end organ and travel in opposite 

 directions to the ends of the muscle. This change propagates itself in 

 a wave-like manner, and has been termed the contraction wave. If 

 a stimulus be applied directly to the end of a long muscle, the con- 

 traction wave passes along its entire length to the opposite extremity, 

 in virtue of the conductivity of muscular tissue. The rapidity of the 

 propagation varies in different animals in the frog, from three to 

 four meters a second, in man, from ten to thirteen meters. The 

 length of the wave varies from 200 to 400 millimeters. 



Graphic Record of a Muscle Contraction. The changes in the 

 form of a muscle during contraction and relaxation have been care- 

 fully studied by recording the muscle movement by means of an 

 attached lever, the end of which is allowed to rest upon a moving 

 surface. The time relations of all phases of the muscular movement 

 are obtained by placing beneath the lever a pen atached to an electro- 

 magnet thrown into action by a tuning-fork vibrating in hundredths 

 of a second. A marking lever records simultaneously the moment of 

 stimulation. 



Single Contraction. When a single electric induction shock is 

 applied to a nerve close to the muscle, the latter undergoes a quick 

 pulsation, speedily returning to its former condition. As shown by 



