OBSEEVATIONS ON STRIPED AND UNSTRIPED MUSCLE. 99 



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 

 considerations 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. 



(iii) The only force which can keep the bars on the stretch 

 must be that of the transverse networks. On this supposition 

 the uncontracted 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 diffi- 

 cult 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 conclusion 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 ceil, and this we 



