OBSERVATIONS ON STRIPED AND UNSTRIPED MUSCLE. 95 



expect to be differentiated first. In the embryonic development of 

 striped muscle it is found that the longitudinal striation appears 

 first, i.e., that the longitudinal bars of the network are differentiated 

 before the transverse. This is also the case in regenerating muscle. 

 Again, in tracing the phylogeny of muscle, we found that the first 

 indication of an intracellular network was in the vertebrate unstriped 

 muscle in the form of longitudinal bars only. Hence both the 

 phylogeny and the ontogeny of the network favours the view that 

 the longitudinal bars are the contractile part of the cell. 



Action of the Transverse Networks. 



Similarly to the longitudinal bars the transverse networks always 

 remain straight in all conditions of contraction and relaxation of the fibre. 

 Hence they become necessarily extended when the muscle-fibre 

 contracts, and return to their original form on relaxation of the 

 fibre. The cruestion now remains as to whether the return of the 

 transverse networks to their original position is due to active contrac- 

 tility or to elastic rebound. The following arguments, for the first of 

 which I am indebted to Mr. Melland, are in favour of the latter 

 view. 



(a) An elastic thread, if stretched and then allowed to rebound, 

 will always return to its original length, i.e., will always shorten to 

 the same extent. The transverse networks behave in this way; they 

 always shorten to the same extent, viz., to the normal diameter of 

 the fibre. This speaks in favour of their being passively elastic, for 

 if they were actively contractile there is no reason why the fibre 

 should not be compressed to less than its normal diameter, elonga- 

 tion at the same time taking place; whereas the fibre always 

 relaxes to the same extent. 



(b) If the statements of Gerlach, Retzius, and Bremer are correct, 

 both parts of the network are connected with the end plate and with 

 the axis cylinder of the nerve, the longitudinal bars being connected 

 indirectly through the transverse networks, the latter being in direct 

 connection with the nerve. It is therefore difficult to conceive that 

 the transverse networks can contract actively after the longitudinal 

 bars have begun to relax, for the nervous impulse will apparently 

 reach the former first, and hence they must contract at tho same 

 time ix.i or b'fure the longitudinal bars ; and yet if the relaxation of 



