Isolated congenital absence of the inferior rectus muscle is rare. So great an authority as Bielschowsky1 stated in a personal communication : "I cannot remember having seen a single case of an isolated congenital paralysis of the inferior rectus muscle. . . ." Further, I have been able to find less than 10 cases reported in the literature.
Whitnall,2 on the basis of his experience in the dissecting room, expressed the belief that deficiencies and abnormalities of the ocular muscles are not as rare as would appear from a survey of the literature. He further stated that they arise from developmental errors in the cleavage from the common premuscular mesoblastic mass. In this, both Whitnall and Mann agreed. Peter3 made the same generalizations.
According to Mann,4 the four rectus and the two oblique muscles separate from their common mesodermal cone, which surrounds the back of the globe at the 20