GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 117 



still more rapid rates of excitation these alterations in the contraction curve 

 are not only exaggerated, but develop more quickly, and play a very 

 important part in producing the peculiar form of continued contraction 

 known as tetanus. 



Lee l states that the slowing of the contraction process, which is compara- 

 tively slight in the muscles of the frog, is very marked in the muscles of the 

 turtle, but practically absent from the white muscles of the cat. Moreover, 

 the prolongation of the relaxation which is very noticeable in the case of the 

 muscles of frogs and turtles, is very slight in those of the cat. Contracture 

 effects have, however, been seen on both the red and pale muscles of the 

 rabbit and on the muscles of man. Although the muscles of different animals 

 show certain peculiarities, the facts illustrated in the above experiments can 

 be considered as in general true of most striated muscles. 



(3) Effect of Frequent Excitations to Produce Tetanus. — As we have seen, the 

 normal muscle the first time that it is excited relaxes almost as quickly as it 

 contracts, but if it be excited rhythmically a number of times a minute, gradu- 

 ally loses its power of rapid relaxation. The tendency to remain contracted 

 begins to show itself in a prolongation of the crest of the contraction curve, 

 even before fatigue comes on, and increases for a considerable time in spite of 

 the effect of fatigue in lessening the height of the contractions. If a skeletal mus- 

 cle of a frog be excited many times, at a rate of about once every two seconds, 

 the gradual increase in the duration of the contractions will have the effect of 

 preventing the muscle from returning to its normal length in the intervals be- 

 tween the succeeding stimuli, for contraction will be excited before relaxation 

 is complete. As is shown in the record of the experiment reproduced in Figure 

 49, there will come a time in the work when the base-line connecting the lower 

 extremities of the succeeding myograms will be seen to rise in the form of a 

 curve, the change being at first gradual, then more and more rapid, and then 

 again gradual (see b, Fig. 49). The effect of the change in the power to relax 

 is to make it appear as if the muscle were the seat of two contraction processes, 

 the one acting continuously, the other intermittently in response to the suc- 

 cessive excitations. Such a condition as that exhibited in section c, Figure 1!'. 

 is spoken of as an incomplete tetanus, complete tetanus being a condition of 

 continuous contraction caused by rhythmical excitations, in which none of the 

 separate contraction movements are visible. In complete tetanus the muscle 

 writes an unbroken curve. 



The slowing of the relaxation of the muscle and consequent state of con- 

 tinued shortening which is to be seen in the latter part of the above experiment 

 is the result of the developing contracture. The amount of contracture 

 increases, within limits, with the increase in the strength and rate of exci- 

 tation. The intensity and rate of stimulation required for the production 

 of this condition depend very largely upon the character of the muscle and 

 its condition at tin' time. In the experiment recorded in Figure 50 the 

 development of the condition of contracture was more marked than in the 

 1 Lee : American Journal of I'hytiology, 1899, ii. 3, p. 11. 



