OBSERVATIONS ON STRIPED AND UNSTRIPED MUSCLE. 99 



to speak, the interstices of the more durable framework, and the 

 metabolism of which alone is large and rapid." 



Summary. 



1. In all muscles which have to perform rapid and frequent move- 

 ments, a certain portion of the muscle is differentiated to perform the 

 function of contraction, and this portion takes on the form of a very 

 regular and highly modified intracellular network. 



2. This network, by its regular arrangement, gives rise to certain 

 optical effects, which cause the peculiar appearances of striped 

 muscle. 



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

 by the active contraction of the longitudinal fibrils of the intracellular 

 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 longitudinal fibrils have ceased to con- 

 tract ; they are possibly also paths for the nervous impulse. 



4. In some cases where muscle has been hitherto described as 

 striped, but gives no appearance of the network on treatment with 

 the gold and other methods, the apparent striation is due to optical 

 effects caused by a corrugated outline in the fibre. 



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

 contraction is comparatively slow and peristaltic in nature, this 

 peculiar network is not developed. In most if not all of the inver- 

 tebrate unstriped muscle there does not appear to be an intracellular 

 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 network intermediate between the 

 typical irregular intracellular 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. 



The investigations connected with this paper were partly carried 

 on in the laboratories of the Owens College and partly at the 

 Scottish Marine Station at Granton. I must here express my 

 thanks to my brother, Professor Milnes Marshall, for his kindness in 

 revising the paper, for much advice in its production, and for 

 obtaining the literature of the subject ; all the controversial points 



