. Electro-physiology. Electrophysiology. 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 spind
. Electro-physiology. Electrophysiology. 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 figur
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