GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 103 



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

 detected less than 0.001 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 Mould 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 cau be observed. In the 

 case of the striated muscles of a frog the electrical latent period is less 

 than 0.001 second, and the mechanical about 0.004 second. Mendelssohn ' 

 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 2 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 

 muscle-cells, fibres as they are called, arranged side by side in bundles, the 

 whole being bound together by a fine connective-tissue network. Each muscle- 

 fibre consists (if a very delicate elastic sheath, the sarcoleimna, which is com- 

 pletely filled with the muscle-substance. Under the microscope the fibres are 

 seen to be striped by alternating light and dark transverse bands, and on focus- 

 ing, the difference in texture which this suggests is found to extend through 



1 Archives de Physiologic, 1880, 2d series, t. vii. p. 197. 



2 Untersuchungen iiber den Erregungsvorgang im Nen>en und Muxkelsystem, 1871. 



