28 ELECTRO-PHYSIOLOGY CHAP. 



the primitive bundle (sarcoplasm with nuclei and fibrils) ; it can, 

 therefore, only be seen clearly in places where, from rupture of 

 the fibres, or any other cause, the contents of the muscular 

 sheath have shrunk back from the wall, or where, conversely, the 

 latter has risen up in a bladder (imbibition of water). The 

 sarcolemma, which Kolliker reckons in vertebrates as a true cell- 

 membrane, may be seen even at an early stage of development 

 in the muscle -fibres as a very delicate integument; other 

 authors regard it as a product of the connective tissue that 

 surrounds the muscle-cell. 



In accordance with the usual disposition, multinuclear 

 muscle-fibres severally exhibit more or less distinct transverse 

 striae, at right angles, i.e., to the axis of the fibre, a characteristic 

 common also to many uninuclear muscle-cells of invertebrates 

 and vertebrates, and due, as will be shown, to the same cause in 

 both instances. 



In addition to the transverse striae, a longitudinal striation 

 (derived from the fibrillated structure) is nearly always apparent ; 

 it may be excessively fine, or may separate the fibres into com- 

 paratively large bundles. 



The relative distinctness of the transverse and longitudinal 

 striae at a given moment varies very considerably in different 

 muscle-fibres. In many cases the cross-stria tion is hardly visible, 

 while the longitudinal striae are clearly marked ; at other times 

 the opposite appears. Different parts, indeed, of one and the 

 same fibre may vary in this particular. This is essentially 

 dependent upon the relation between the contractile fibrils and 

 the interfibrillar sarcoplasm, which (as many recent observations 

 have established) may vary enormously, alike in the muscles of 

 different animals, and in the different muscles of the same species. 

 As was stated above, the examination of cross-sections gives the 

 best and surest conclusions. The salient feature under all 

 conditions is, in the fully-developed fibre also, the unequal dis- 

 tribution over the surface of the section. The fibrils are arranged 

 in larger or smaller bundles ("muscle-columns"), separated by 

 more or less bulky discs (striae, from the longitudinal aspect) of 

 sarcoplasm (interstitial substance, interfibrillar substance, sarcogleia), 

 as in so many smooth and cross -striated uninuclear muscle- 

 cells. The more abundant the sarcoplasm, the easier the division 

 of a primitive bundle into " muscle-columns," i.e. bundles of 



