The structure of the sarcotubular system in avian muscle

Most avian skeletal muscles have their triads at the A‐I junction, and there is a complex sarcoplasmic reticulum in the Z‐line region because of the architectural arrangement of two triads in one sarcomere. The sarcotubular system in the pigeon serratus superficialis metapatagialis muscle was examined with the electron microscope. In these muscles the t‐tubules open to the extra‐cellular space, and there is an indication that mitochondria may connect to the sarcotubular system. The elements of the sarcoplasmic reticulum between two triads of a single sarcomere is similar to that in the frog, and consists of the triads, intermediate cisternae, longitudinal tubules, and fenestrated collar. In the portion of the sarcoplasmic reticulum between triads of adjacent sarcomeres, the triads are coupled. This is brought about by triads extending from one sarcomere to the next and by the connection of the reticulum between triads of adjacent sarcomeres. Some of the tubules making up the Z‐line reticulum have a fenestrated appearance, similar to the fenestrated collar.