94 c. f. marshall. 



Action op 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 tliat they always remain straight in all conditions of contraction and 

 relaxation of the fibre. The question now before us is to determine if 

 they are actively contractile or passively elastic. The following con- 

 siderations are opposed to the latter view : — 



(a) If the longitudinal bars of the network are passively elastic, 

 they must be on the stretch in the relaxed condition of the fibre, 

 and resemble stretched elastic threads running the whole length 

 of the muscle-fibre. Now, when a muscle is cut out of the body, and 

 thereby removed from its attachments, it does not contract to any 

 considerable extent ; therefore, supposing the longitudinal bars to be 

 elastic, something must keep them on the stretch. 



(i.) This cannot be the sarcous substance, for as it is semi-fluid in 

 nature it can hardly keep elastic threads on the stretch. 



(ii.) It cannot be a nervous impulse, continually acting on the 

 longitudinal bars, for if it were so a muscle would contract on section 

 of its nerve. 



(in.) The only force which can keep the bars on the stretch must 

 be that of the transverse networks. On this supposition the uncon- 

 tracted muscle is not in a state of rest, for there is a continual force 

 exerted against the transverse networks by the tendency of the 

 longitudinal bars to shorten. It is very difficult to conceive that the 

 muscle, in its uncontracted condition, should be in a state of extreme 

 tension, and not of comparative rest. 



(b) In the unstriped muscle-fibre there are no transverse networks 

 present, and hence no force to keep the longitudinal fibrils on the 

 stretch, except the sarcolemma, which would be scarcely adequate to 

 do so. 



It therefore appears improbable that the longitudinal bars of the 

 network are passively elastic, and if this is the case the only con- 

 clusion remaining is that they are actively contractile, and hence, 

 presumably, the cause of contraction of the fibre. This view is also 

 supported by the following considerations : — 



In the muscle-cell the part which performs the contraction is 

 evidently the most fundamental part of the cell, and this we should 



