A cross-section of the tubular heart of Limulus shows the vast majority of the fibers cut longitudinally. They are collected into larger and smaller bundles, arranged approximately radially, tangen tially, and circularly, crossing each other at angles of all degrees; a very few scattered cross-cut fibers may appear peripherally and cen trally. The vast majority of the fibers are thus transversely disposed, only an insignificant moiety longitudinally. A median longitudinal section, on the contrary, shows a majority of the fibers cut transversely. Between these, and separating them into wider layers, are narrower bundles of longitudinally cut fibers, the radial fibers. One may search many sections cut near the medial longitudinal plane without seeing a single longitudinal fiber. The latter are certainly extremely rare, but a very few are apparently present, scattered peripherally and next the lumen. Both transverse and longitudinal sections immediately dis close an intricate loose-meshed muscular syncytium.
Figure Se illustrates a cross-section under low magnification (1,000 diameters) of a large fiber at the level where it has become resolved into several main and a number of secondary branches. Each branch is enveloped by a delicate sarcolemma, the peripheral portion fusing intimately with the endomysium which incloses the entire bundle and separates the main branches. For purposes of minuter description a smaller trabecula, or branch, may be used. The myofibrilhe are grouped into large masses, in which they are arranged in lamella (fig. 6s) undergoing a peripheral radial and a central vertical splitting.
The nuclei are located both centrally and peripherally.
Figure 7 illustrates the typical appearance of a small fiber. The myofibrillte are segregated centrally and are enveloped by a consider able area of granular cytoplasm confined by a delicate sarcolemma. The nucleus here lies wholly in the extrafibrillar sarcoplasm. Upon close inspection the lamellae are seen to consist of fibrillae radially arranged. The apparent lamellae illustrated in figure 6s are therefore smaller groups of fibrilla).
As in the skeletal muscle, the cardiac fiber consists of peripheral lamellar and central cylindrical groups of fibrillae. The close similarity between the cardiac and the skeletal muscle extends to the structure and distribution of the nuclei, the finer resolution of the apparent elementary fibrillae (fig. 8), and the presence of a true cell-membrane or sarcolemma.
In longitudinal sections the appearance of the fiber again, as in the skeletal muscle, and almost identically, varies according to the degree of contraction. Certain fibers (fig. 9) show a wide darker-staining Q-disk and an alternating slightly wider J-disk bisected by a deep staining granular Z-membrane. In certain other fibers, also uncon tracted, only the deep-staining granular Z-membrane is conspicuous (fig. 11). The contracted fibers (fig 13c) show only a succession of contraction bands, alternating with lighter-staining disks. The con traction bands are very similar to those of the skeletal muscle and probably likewise represent a thickened Z-membrane, due to a segre gation of Q-substance about them.