GENEKAL PHYSIOLOGY OF MUSCLE 9 



slowly, then more rapidly, then more slowly again, which repre- 

 sents the contraction period. 



(c) The interval c d, during which the muscle relaxes and 

 lengthens, slowly at first, then more rapidly, then again slowly, 

 which is the expansion or elongation period. 



According to Helmholtz' first results the latent period in the 

 voluntary muscles of the frog is about 0*01 sec., but later work has 

 shown it to be much shorter. According to Yeo it is O005 sec. ; 

 according to Burdon-Sanderson 0*0025 sec. ; lastly, according to 

 Tigerstedt (who made many comparative experiments on the 

 frog's gastrocnemius under a variety of conditions) it varies between 

 0*004 to 0-006 sec., but is generally (41 per cent) 0'005 sec. 



From the theoretical standpoint it is more than probable that 

 there is really no appreciable interval between the direct stimula- 

 tion of a muscle and the commencement of contraction, and that 

 the apparent latency of excitation depends on the fact that the 

 contraction does not begin simultaneously throughout the mass of 

 the muscle, but advances gradually like a wave, so that the fibres 

 which first contract pull upon, and passively extend, the fibres 

 that have not yet contracted, and thus nullify the mechanical 

 effect. It is only when, with the advance of the contraction wave, 

 the active shortening of the mass of muscle exceeds its passive 

 elongation that the lever attached to the muscle begins to rise 

 from the abscissa (Gad). 



Apart from the latent period, the active reaction or excitation 

 of the muscle consists in a diphasic process, with distinct phases 

 of contraction and of expansion, which may vary considerably 

 under different circumstances. For instance : 



(ft) Tracings of a muscle twitch vary considerably in the 

 duration or velocity of the total movement and that of the two 

 separate phases, according to the character of the muscles which 

 are under observation. As regards speed of reaction, there is an 

 enormous difference between the plain muscles, which react so 

 slowly that both phases are visible to the eye, and the striated 

 muscles, which react so quickly that the graphic method is indis- 

 pensable for their demonstration. The cardiac muscle cells come 

 midway as regards rate of response between the unstriated visceral 

 and the striated skeletal muscles. The duration of the contraction 

 of skeletal muscles is variable, not only in the muscles of different 

 animals, but even in the different muscles of the same animal. 



Contraction is most rapid in insects, less rapid in birds, slower 

 still in mammals (about 0*1 sec. on "an average), slowest of all in 

 the cold-blooded animals, especially in the tortoise. 



Eanvier (1874) first noted that in certain birds and mammals 

 two kinds of muscles can be distinguished, red and pale, and that 

 the latter contract more rapidly than the former, an important 

 fact subsequently confirmed by other experimenters. 



