THE SARCOPLASMIC RETICULUM OF STRIPED MUSCLE

Dr. Edwards and colleagues have demonstrated that many types of vertebrate and invertebrate muscle possess an extensive representation of the endoplasmic reticulum, and have observed that this may be arranged in longitudinal members running parallel to the myofibrils, and in transverse membranes, often located at the level of the junctions of A and I bands. Dr. Porter has called attention to some earlier work with light and electron microscope which deals with this reticulum, and has supplied us with new details of the structural arrangements prevailing at the points at which the sarcoplasmic reticulum connects with the Z bands of the myofibrils. So far as I have been able to ascertain, the earliest representation of a component of the sarcoplasm which was disposed with a relationship to the crossstriations of muscle is that of Dobie (4), who in 1849 examined preparations wherein the individual myofibrils of fish and amphibia had been teased apart. He observed between the separated myofibrils, " . . . a beautiful homogeneous membrane (resembling the web between two of the toes of a duck) which is stretched by the violence used in the separation of the fibrillae." This structure in the skate, " . . . was marked with stripes corresponding to the dark and light spaces of the fibrillae between which it was stretched." The first definitive visualization of the sarcoplasmic reticulum can be attributed to Thin (14), who impregnated it with gold and observed transverse and longitudinal components, but who misunderstood its significance. Melland (7), in confirming these observations in 1885, extended them by noting a fragment of isolated sarcolemma, to the inner surface of which were adherent components of the reticulum in the forms of circumferential lines at levels corresponding to those of the Z bands. Shortly thereafter a more detailed description of the reticulum was presented by Cajal (3), who observed the sarcoplasmic reticulum in living unstained insect muscle, but who erroneously interpreted the reticulum as constituting the active contractile units of the fiber. Other descriptions of importance were published by Retzius (12, 13), Marshall (6), and others. But the most extensive and valuable description of the sarcoplasmic reticulum as seen with the light microscope is that of Veratti (15). This pupil of Golgi prepared elegant metallic impregnations of the reticulum in muscles of various vertebrates and invertebrates, and published his observations with colored 171