66 0. F. MARSHALL. 



3. The contraction of the striped muscle- fibre is probably 

 caused by the active contraction of the longitudinal fibrils 

 of the intra-cellular network : the transverse networks appear 

 to be passively elastic, and by their elastic rebound cause the 

 muscle to rapidly resume its relaxed condition when the longi- 

 tudinal fibrils have ceased to contract; they are possibly also 

 paths for the nervous impulses. 



4. In some cases where muscle has been hitherto described 

 as striped, but gives no appearance of the netAvork on treat- 

 ment with the gold or other methods, the apparent striation is 

 due to optical effects caused by a corrugated outline to the 

 fibre. 



5. In muscles which do not perform rapid movements, but 

 where contraction is comparatively slow and peristaltic in 

 nature, this peculiar network is not developed. In most if not 

 all of the invertebrate unstriped muscle there does not appear 

 to be an intra-cellular network present in any form ; but in the 

 vertebrate unstriped muscle a network is present in the form of 

 longitudinal fibrils only ; this possibly represents a form of net- 

 work intermediate between the typical irregular intra-cellular 

 network of other cells and the highly modified network of 

 striped muscle. 



6. The cardiac muscle-cells contain a network similar to 

 that of ordinary striped muscle. 



Discussion of the Views of Recent Observers. 

 Before commencing the subject-matter of this paper I propose 

 to discuss the results arrived at by several recent observers 

 concerning the structure of muscle-fibre, and also some of the 

 criticisms which have lately appeared concerning the existence 

 of an intra-cellular network in striped muscle. 



The most important paper to discuss is that of Rollett.^ 

 He considers the muscle-fibre to consist of longitudinal fibrilljc 

 grouped together into " muscle-columns," the cross-sections of 

 which correspond to Cohnheim's areas. Filliug up the spaces 

 between the muscle-columns is the iutcrfibrillar material or 

 • 'Arcli. f. mikr. Anat.,' 1SS8, pp. 233—205. 



