26 ELECTRO-PHYSIOLOGY CHAP. 



Generally speaking, therefore, it may be said that, in regard 

 to histological structure, the cardiac muscles exhibit great 

 similarity with certain muscles of the lower animals and the 

 developing stages of cross-striated, multinuclear muscle-fibres, 

 and are thus in a certain sense embryonic in character. This 

 applies not merely to the striking abundance of sarcoplasm, but 

 also to the form and arrangement of the muscle-columns, and 

 central situation of the nucleus. 



The elements of cardiac muscle in Vertebrates, in particular 

 of mammals, together with the cross -striated uninuclear cells 

 of Invertebrates, form the transition to the type of muscle which 

 is, anatomically and physiologically, the most highly developed. 



CROSS-STRIATED, MULTINUCLEAR MUSCLE-FIBRES 



Among Invertebrates these occur in Arthropoda only ; in 

 Vertebrates, collectively, they form the chief bulk of muscle. 



At one time it was supposed, chiefly on account of the 

 multiplicity of nuclei in striped muscle-fibres, that these re- 

 presented a fused series of many cells (a " syncytium "), but it is 

 now admitted that each striated muscle-fibre is equivalent to a single 

 cell, from which, indeed, it has developed. At first these are 

 uninuclear, spindle-shaped cells, which grow rapidly longer, while 

 the nucleus increases by repeated division. Subsequently the 

 elongated, multinuclear spindles become not only longer, but much 

 wider, while a gradual differentiation of striated fibrils is pro- 

 ceeding from the increasing bulk of protoplasm (sarcoplasm). 

 These fibrils, viewed longitudinally, or better in transverse section, 

 do not occupy the entire thickness of the fibre, but arrange them- 

 selves superficially as a tubular sheath, or (in many of the lower 

 vertebrates) as a segment lying to one side, so that the nucleated, 

 formative (sarco-) plasma lies, as it were, enclosed in a canal or 

 furrow (Fig. 17). 



A temporary relation thus arises between the latter and the 

 differentiated fibrils, which is altogether analogous to the constant 

 distribution of the permanently uninuclear myoblasts of most 

 invertebrates. As development progresses the peripheral layer 

 of fibrils, at. first extremely attenuated, increases in bulk at 

 the expense of the sarcoplasm, so that eventually the ratio is 



