94 THE HUMAN BODY 



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

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

 mences in spite of the stimulation. When our muscles are stimu- 

 lated in the Body, from the nerve-centers 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 muscle varies 

 with the stimulus. In the first place, a single stimulus 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 in- 

 fluence, a greater stimulus causing a greater contraction; but very 

 soon a point is reached beyond which increase of stimulus 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 (^ inch). Then if we hang one gram (15.5 grains) on 

 it and give it the same stimulus, it will be found to contract more, 

 say four or five millimeters, and so on, up to the point when it 

 carries eight or ten grams. After that an increased weight will, 

 with the same stimulus, cause a less contraction. So that up to a 

 certain limit, resistance to the shortening of the muscle makes it 

 more able to shorten: the mere greater extension of the muscle 

 due to the greater resistance opposed to its shortening, puts it into 



