Extraocular muscles in the microphthalmic rat.

The extraocular muscles in a mutant microphthalmic strain of rat were studied. The eyeball of this strain of rat is reduced to about a third in diameter of that of the normal rat. Nevertheless, in the orbit of the mutant rat, every one of the extraocular muscles was identified; their origins and courses were the same as in the normal rat, but differences existed in the insertions. These insertions could be classified into three groups: Group A (retractor bulbi): like normal insertion into the eyeball. Group B (superior rectus and superior oblique): attachment of tendonlike insertions to each other; these muscles come from opposite directions and form a loop. Group C (lateral, medial, and inferior rectus and inferior oblique): insertion into connective tissue surrounding the reduced eyeball. The volume of each muscle of the mutant rat was smaller than that of the normal rat; moreover, significant differences existed in the degree of reduction in the volume of each muscle group classified according to the change of insertion. In the group A muscle the volume was only 33% of the normal volume, whereas group B was 74% and group C was about half of normal.