102 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



that the muscle protoplasm becomes active before any outward manifestation 

 occurs. That this view is correct has been proved by electrical observations. 



When muscle protoplasm passes from a state of rest to one of action it 

 undergoes an alteration in electrical condition. This change can be detected by 

 the galvanometer (Fig. 58, p. 135) or by the capillary electrometer (Fig. 59, 

 p. 136). Burdon Sanderson 1 has found that by the aid of the latter instru- 

 ment an alteration of the electrical condition of the muscle of a frog can be 

 detected within 0.0025 second after the stimulus has been applied to it. Since 

 some slight interval of time must have been lost even by this delicate method, 

 it would seem that muscle protoplasm begins to be active at the instant it is 

 stimulated. 



According to this view, muscle-substance has no latent period ; neverthe- 

 less we can still speak of the latent period of the muscle as a whole. It will 

 be necessary, however, to distinguish between the electrical latent period and 

 the mechanical latent period : by the former we mean the time which elapses 

 between the moment of excitation and the first evidence obtainable of a change 

 in the electrical condition of the muscle ; by the latter, the time between exci- 

 tation and the earliest evidence of movement which can be observed. In the 

 case of the striated muscles of a frog the electrical latent period is about 0.0025 

 second, and the mechanical about 0.004 second. Mendelssohn 2 estimated the 

 mechanical latent period of the muscles of man to be about 0.008 second. 

 There can be little doubt, however, that this figure is too large. 



Bernstein 3 found that if a normal frog's muscle be excited indirectly, 

 by the stimulation of its nerve, the mechanical latent period is somewhat 

 longer than when it is directly excited. Of course a certain length of time is 

 required to transmit the excitation through the length of nerve intervening 

 between the point stimulated and the muscle fibres. If this time be deducted, 

 there still remains a balance of about 0.003 second, which can only be ac- 

 counted for on the assumption that the motor nerve end-plates require time to 

 excite the muscle-fibres. The. motor end-plates are therefore said to have a 

 latent period of 0.002-0.003 second. 



The latent period, and the time required for the rise and fall of the myo- 

 graph curve, are found to be very different not only for the muscles of differ- 

 ent animals, but even for the different muscles of the same animal. Moreover, 

 the time relations of the contraction process in each muscle are altered by a 

 great variety of conditions. 



Before considering the effect of various influences upon the character of the 

 muscle contraction, let us give a glance at the finer structure of the muscle, 

 and the change of form which the microscopic segments of the muscle-fibre 

 undergo during contraction. 



2. Optical Properties of Striated Muscle during Rest and Action. 

 An ordinary striated muscle is composed of a great number of very long 



1 Centralblatt filr Physiologic, July 5, 1890, vol. iv. 



2 Archiv de Physiologic, 1880, 2d series, vol. vii. p. 197. 



3 Untersuchungen iiber den Erregungsvorgang im Nerven und Muskelsystem, 1871. 



