98 PRINCIPLES OF ANIMAL BIOLOGY 



and its responses are weaker. Smooth muscle, as in the intestine, reacts 

 much more slowly, the contraction lasting about 20 seconds. The relax- 

 ation of any muscle is purely passive; the ends of the muscle fibers do 

 not push. 



Single twitches are not, however, the commonly observed type of 

 muscle action. During ordinary contraction, nerve impulses are deliv- 

 ered in rapid succession, beginning, say, at 4 or 5 per second and increas- 

 ing in frequency to 40 or 50 per second. These rapidly repeated stimuli 

 may be shown experimentally to be the most effective method of getting 

 strong and sustained contraction. The nature of the contraction result- 

 ing from stimuli repeated at different rates is shown in Fig. 87. In the 

 lowest curve the stimuli were given at a slow rate, and after each one 

 the muscle relaxed almost to its former state. But when the stimuli 

 were given more and more rapidly, as in the remaining curves of the 

 figure, complete relaxation did not have time to occur between them, 

 and the total contraction gradually increased. 



In striated muscle the cells act separately and do not communicate 

 stimuli to surrounding cells. In smooth muscle, however, stimulation 

 at one point may lead to a wave of contraction passing over a whole 

 sheet of muscular tissue, showing that the stimulus is communicated 

 from cell to cell. 



The efficiency of muscle, that is, the ratio of work done to energy 

 consumed, is rather high. For a single twitch, including the recovery 

 period following, this ratio is about 50 per cent. For sustained contrac- 

 tion, however, the efficiency is much less — around 25 per cent. 



Chemistry of Muscle Contraction. — Just what happens in a striated 

 muscle when it contracts is only partially understood. It is the myo- 

 fibrils that do the contracting, but the important thing to know is the 

 set of physical or chemical conditions which cause them to shorten. 

 Clues have been furnished by chemical analysis of fatigued muscle. 

 Most of the glycogen, which in rested muscle amounts to about 3 per 

 cent of the weight, has disappeared in fatigue, as has also much of the 

 oxygen. At the same time the inorganic phosphates (produced out of 

 organic phosphates) have considerably increased; so also has carbon 

 dioxide. If imder experimental conditions oxygen is excluded there is 

 also an increase of lactic acid. How the glycogen is lost is known; 

 combining with water, it is converted into glucose and lactic acid. 

 Something must also have been oxidized to account for the increased 

 carbon dioxide. Under ordinary conditions the lactic acid does not 

 persist, for part of it is oxidized to obtain energy with which the rest 

 of the lactic acid is reconverted to glycogen. Formerly it was thought 

 that the breaking down of glycogen or the oxidation of one of its products 

 furnished the energy for muscle contraction; yet conversion of glycogen 



