ANATOMICAL STUDY OF THE OPOSSUM (DIDELPHIS-ALBIVENTRIS) EXTRAOCULAR-MUSCLES

The anatomy of the extraocular muscles was studied in 10 adult opossum s (Didelphis albiventris) of both sexes. Eight extraocular muscles wer e identified: 4 rectus muscles, 2 oblique muscles, the levator palpebr ae superioris and the retractor ocular bulbi. The rectus muscles origi nate very close one to another between the orbital surfaces of the pre sphenoid and palatine bones. These muscles diverge on the way to their insertion which occurs at about 2 mm from the limbus. The levator pal pebrae superioris originates with the dorsal rectus and is positioned dorsally in relation to it. The retractor ocular bulbi forms a cone wh ich embraces the optic nerve and is located internally in relation to the rectus muscles. The dorsal oblique originates on the presphenoid b one and after a tendinous trajectory through a trochlea on the medial wall of the orbit, inserts into the ocular bulb. The only muscle arisi ng from the anterior orbital floor is the ventral oblique. The main ne rve supply for these muscles is the oculomotor, except for the dorsal oblique which is innervated by the trochlear nerve, and the lateral re ctus which is innervated by the abducens nerve. The retractor ocular b ulbi receives branches from the inferior division of the oculomotor ne rve and some branches from the abducens nerve.