Comparative anatomy of the extrinsic ocular muscles in vertebrates

Comparative anatomy of the orbita throughout various vertebrates revealed that the extrinsic ocular muscles vary in shape and number, and it was suggested that the original form of the 3 straight (superior, inferior, and temporalis) and one oblique (superior) muscles serve to conduct their components into various types of differentiation and degeneration. Differentiation of the extrinsic ocular muscles from 4 origins seems to depend both on early independence of the M. obliquus inferior and M. rectus nasalis leading to the primitive pattern of Cyclostomata and Pisces, and new gradual development of the M. retractor bulbi, M. membranae nictitans, and M. palpebralis toward Amphibia and Amniota, in order to move the bulbus and eyelid more complicately. Degeneration, on the contrary, implied partial reduction of original muscles together with their nerves and complete disappearance of them. Occasional division of the M. obliquus superior and M. palpebralis into 2 parts with the same innervation, moreover, suggests the probable appearance of new muscles in relation to eye movement and in animal species not yet examined. These changes, including the original form, primitive pattern, and most differentiated types, were summarized schematically in 27 species of vertebrates.