1 88 HUMAN PHYSIOLOGY 



2. The external phenomena during the transformation 

 of energy in an active muscle. The transformation of 

 energy reveals itself in definite mechanical, thermal, and 

 electrical changes in the muscles. 

 A. Mechanical changes in the stimulated muscle. 



(a) Contraction. The stimulated muscle shortens in its 

 longitudinal direction, the diameter increases, while the 

 volume remains the same. 



Both the anisotropic and the isotropic bands of the striated 

 muscle change in the same sense as the whole muscle. 

 That the volume of the anisotropic part is increased, while 

 that of the isotropic part is slightly decreased, is explained by 

 the passing of water from the isotropic to the anistropic part. 

 Besides this, the optical difference between the two parts 

 becomes less. 



Twitching. If a muscle is acted upon by a stimulus last- 

 ing for but a short time (by an induced electric current), it 

 draws itself together rapidly and then again immediately 

 lengthens. This is called a twitch. 



The length of time consumed by a twitch is investigated by the 

 graphic method. The muscle is connected with a writing-lever 

 which is moved by its contraction and writes its movement on a 

 travelling surface. Such an apparatus for the graphical registra- 

 tion of a muscle contraction is called a myograph. 



An isotonic contraction is a contraction during which the tension 

 (tonus) of the muscle remains constant. The isotonic contraction 

 curve shows the duration of the contraction during constant ten- 

 sion. To obtain such a curve a writing-lever must be used which 

 is thrown upward as little as possible by the contracting muscle. 

 A light lever is used and a weight is hung as near the axis as 

 possible, while the muscle is attached to the lever at considerable 

 distance from the axis. During normal physiological conditions, 

 a muscle does not contract isotonically, but always with change in 

 tension. 



An isometric contraction is a contraction in which the shortening 

 of the muscle is completely prevented, so that tension is produced 

 without the shortening of the muscle. The changes in tension in 

 an isometric contraction can be registered by the so-called tension 

 recorder. 



A noticeable length of time elapses between the moment 

 of stimulation and the beginning of contraction ; this time is 



