Cytochemical differentiation of tonic (slow) and phasic (fast) muscle units in the ocular muscles of a fresh-water carp, Catla catla (Ham.).

1. Functional morphology of the ocular muscles with special reference to their topography, innervation and action have been studied in a fresh-water carp Catla catla. An attempt has also been made to differentiate tonic (slow) and phasic (fast) muscle units in the various ocular muscles by their SDH activities and their lipid contents. 2. Catla catla has large eyes (2 cm in diameter) in a fish measuring 30 cms. There are four recti (inferior, superior, anterior and posterior) and two obliquii (superior and inferior) muscles for the synchronized rotation of the eye ball within the orbit. Anterior and posterior myodomes are well developed. The former accommodates two obliquii muscles, while the latter gives space for the posterior and anterior recti. 3. In this fish, all the ocular muscles are composite in nature being made up of tonic (slow) and phasic (fast) muscle units. The former is made up of red fibers whereas the latter have only white fibers. In the tonic muscle units lipid is more concentrated. 4. The superior and the inferior obliquii muscles rotate the eye ball on its antero-posterior axis. The upward and downward movements of the eye balls are achieved by the superior and the inferior recti. The anterior and posterior recti move the eye ball in the antero-lateral and postero-lateral directions. 5. The superior obliquii contains about 41% of the tonic and 59% of phasic muscle units. The inferior oblique is made up of 25% of tonic and 75% of the phasic muscle units. 6. The Anterior rectus contains greater percentage of tonic muscle fibers (48%) than the superior rectus (40%), the Interior rectus (34%) and the posterior rectus (35%).