96 C. F. MARSHALL. 



the fibre is held to be due to active contraction of the transverse 

 networks, this is what must occur. 



Conclusion. 



The conclusion to which I am therefore led is that the contraction 

 of the striped muscle-fibre is due to the active contraction of the 

 longitudinal bars of the network, and that the transverse networks 

 are probably passively elastic, and by their rebound cause relaxation 

 of the muscle-fibre. That the transverse networks and the muscle 

 corpuscles, with which they are said to be continuous, possibly 

 furnish paths by which the nervous impulse is conveyed from the 

 nerve-ending to the longitudinal bars. That the contraction of the 

 unstriped muscle-fibre is due to the active contraction of its longi- 

 tudinal fibrils when these are present, as in vertebrate muscle. In 

 the case of unstriped muscle which possesses no fibrils, the contraction 

 is due to the whole protoplasm of the cell, there being no special part 

 differentiated to perform this function. 



Should these conclusions prove to be correct, we may imagine the 

 changes that occur in the striped muscle-fibre during contraction to 

 be as follows : — 



The nervous impulse reaching the end-plate of the nerve is 

 conducted by the transverse networks to the longitudinal bars, and 

 causes them to shorten ; it does not cause the transverse networks 

 to contract, because they are passively elastic and non-contractile. 

 The longitudinal bars shorten according to the strength of the 

 nervous impulse, and remain so as long as it lasts. By fluid 

 pressure the transverse networks are extended, and remain so as long 

 as the longitudinal bars remain contracted ; when these cease to 

 contract the elasticity of the transverse networks comes into play, 

 and they shorten to their original dimensions, and by fluid pressure 

 extend the longitudinal fibrils to their original length, the elastic 

 sarcolemma aiding in the process. 



The alternate action of the longitudinal and transverse networks 

 no doubt causes the special features of the contraction of striped 

 muscle, viz., the quick response to stimulus and the rapid contrac- 

 tion ; and we have seen that the network is developed wherever 

 rapid movements have to be performed. 



In connection with the foregoing considerations, the results of 



