134 THE HUMAN BODY. 



ing under the influence of the second stimulus a third electric 

 shock will make it contract more, and so on, until the muscle 

 is shortened as much as is possible to it for that strength of 

 stimulus. If now the stimuli be repeated at the proper in- 

 tervals, each new one will not produce any further shortening, 

 but, each acting on the muscle before the effect of the last 

 has begun to pass off, the muscle will be kept in a state of 

 permanent or "tetanic" contraction; and this can be main- 

 tained, by continuing the application of the stimuli, until the 

 organ begins to get exhausted or " fatigued "; elongation then 

 commences in spite of the stimulation. When our muscles 

 are stimulated in the Body, from the nerve-centres through 

 the nerves, they receive from the latter a sufficient number 

 of stimuli in a second (the exact number is still doubtful) to 

 throw them into tetanic contractions. In other words, not 

 even in the most rapid movements of the Body is a muscle 

 made to execute a simple muscular contraction; it is always 

 a longer or a shorter tetanus. When very quick movements 

 are executed, as in performing rapid passages on the piano, 

 the result, is obtained by contracting two opposing muscles 

 and alternately strengthening and weakening a little the 

 tetanus of each. 



Causes affecting the Degree of Muscular Contraction. 

 The extent of shortening which can be called forth in a mus- 

 cle varies with the stimulus. In the first place, a single stim- N 

 ulus can never cause a muscle to contract as much as rapidly 

 repeated stimuli of the same strength since in the latter 

 case we get, as already explained, several simple contractions 

 such as a single stimulus would call forth, piled on the top 

 of one another. With powerful repeated electrical stimuli 

 a muscle can be made to shorten to one third of its resting 

 length, but in the Body the strongest effort of the Will never 

 produces a contraction of that extent. Apart from the rate 

 of stimulation, the strength of the stimulus has some influ- 

 ence, a greater stimulus causing a greater contraction; but 

 very soon a point is reached beyond which increase of stimu- 

 lus produces no increased contraction; the muscle has reached 

 its limit. The amount of load carried by the muscle (or the 

 resistance opposed to its shortening) has also an influence, 

 and that in a very remarkable way. Suppose we have a frog's 

 calf-muscle, carrying no weight, and find that with a stimulus 

 of a certain strength it shortens two millimeters ( T ^ inch). 



