98 0. F. MARSHALL. 



and unstriped muscle respectively are the great length of the 

 latent period and the long duration of the contraction in the 

 unstriped muscle. The velocity of the contraction-wave in 

 striped muscle is in the Frog, 3 — 4 metres per second, while 

 in the unstriped muscle (ureter) the velocity is only 20 — 30 

 mm. per second. 1 This seems to indicate that the peculiarly 

 arranged network of striped muscle may be associated with 

 the rapidity of its contraction. 



In nearly all the specimens I have examined both the 

 transverse and longitudinal bars of the network 

 remain perfectly straight in all conditions of con- 

 traction and relaxation of the muscle. Hence the 

 network, or part of the network, must either con- 

 tract to the full extent that the muscle-fibre does, 

 or else be elastic and so follow the movements of 

 contraction of the fibre. 



Retzius 2 figures a specimen in which the longitudinal bars 

 are zigzag. However, from his description, and from com- 

 parison with my own preparations, I believe this to be due to 

 disturbance during the preparation and not to be a normal 

 condition. 



We have now to consider whether the network is actively 

 contractile or merely a passively elastic structure ; or whether 

 one part of it is contractile and the other passive. That both 

 network and sarcous substance are contractile is improbable ; 

 for if the function of the network and the sarcous substance is 

 identical, there is no apparent reason for the presence of the 

 network. Differentiation in structure always implies differen- 

 tiation in function. 



Action of the Longitudinal Bars of the Network. 



We have seen that the longitudinal bars of the network 

 diminish in length and apparently increase in thickness during 

 contraction, and that they always remain straight in al|l 



1 'Text-book of Physiology,' Dr. Michael Foster, 4th ed., p. 101. 



2 Loc. cit., plate i, fig. 19. 



