ORGANISATION AND STRUCTURE OF MUSCLE 



25 



The arrangement of the cross - striated fibrils within the 

 formative plasma (sarcoplasm) is very remarkable. The sarco- 

 plasm is so abundant both in vertebrate and invertebrate animals, 

 that the otherwise non-membranous cardiac muscle-cells come 

 under the category of " dark " muscles, with which their 

 easy separation into fibrils and bundles of fibrils also coincides. 

 In transverse section the cardiac muscle -cells of Fishes and 

 Amphibia exactly carry on the type of Cephalopoda and Gastro- 

 poda, each spindle -cell exhibiting a richly developed, central, 

 medullary substance surrounding the nucleus, and enclosed in its 

 turn by the transversely striated fibrils of the cortical substance, 

 which are often arranged in radial strata. 



The heart of Eeptiles and Birds is characterised by the same 

 structure; the contractile cortex of the latter in particular 

 frequently exhibits distinct radial 

 striation. In Mammals also the 

 elements of the cardiac muscle 

 (which is here, as in other verte- 

 brates, of a deep red colour) 

 are among the most richly proto- 

 plasmic parts of the body. The 

 distribution of sarcoplasm and 

 fibrils closely resembles the 

 spindle-cells of Salpa described 

 above ; the bundles of fibrils (Kolliker's " muscle-columns ") not 

 only form a peripheral cortical zone, but are developed within the 

 central nucleated axial plasma (Fig. 16). The plasma itself is 

 usually accumulated round the nucleus, which lies somewhat 

 toward the centre of each cell. The bundles of fibrils (muscle- 

 columns) present considerable variations of form and arrangement 

 in different mammals. The radial, band-shaped bundles of fibrils, 

 striped in cross-section, with which we are so familiar in Inverte- 

 brates, and also in certain muscles of the lower Vertebrata, but 

 which appear only in the heart of Mammals (dog, porpoise), are 

 very frequent. Often, besides these band-shaped muscle-columns, 

 which figure in transverse section as a radially striated, laminar, 

 cortical zone, another prismatic section rounded or polygonal 

 appears in the centre of the muscle-cell (dog, man) (Fig. 16). 

 And, in conclusion, there are many examples (pig) in which these 

 last only are present. 



FIG. 16. Transverse section of cardiac 

 muscle-cells Man. (Kolliker.) 



