AMPHIBIA. 209 



are those of Melland and C. F. Marshall, as set forth in the 

 following summary given by the latter : 



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

 movements, 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 pas- 

 sively elastic, and by their elastic rebound cause the muscle to 

 rapidly resume its relaxed condition when the longitudinal fibrils 

 have ceased to contract ; they are possibly also paths for the 

 nervous impulse. 



"4. In some cases where muscle has hitherto been 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 

 invertebrate unstriped muscles 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 inter- 

 mediate 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 motor nerve-fibres may terminate in end-plates (compare 

 Fig. 90), within the sarcolemma of striated muscle-fibres, but 

 more frequently the axis-cylinder breaks into a brush of fibrils 

 which run longitudinally in the muscle- substance. 



10. The nervous system (Fig. 51) may conveniently be sub- 

 divided into (1) cerebro-spinal axis; (2) cranio-spinal nerves ; 

 and (3) sympathetic system. 



(1) The cerebro-spinal axis is made up of the brain and spinal 

 2 14 



