Observations on the muscles of the eye of the bowhead whale, Balaena mysticetus.

Abstract

The muscles of the eyelids and the extraocular muscles of mysticete whales are poorly described for a variety of reasons, including considerable difficulty in obtaining specimens. Our objective is to provide such a description for the bowhead whale, Balaena mysticetus. This study has examined the gross anatomy of the region in six specimens (five adults, one fetus) of the bowhead whale. Results show that the muscles associated with the eye are well developed and possess several distinctive features. For example, precise limits of each extraocular muscle are difficult to determine along their entire length because these muscles intermingle with one another near their insertion. Furthermore, some fibers from these muscles (except the retractor bulbi) also insert into the eyelids. Pulling on these muscles to simulate contraction results in movement of the eyelids and suggests a role for these muscles in palpebral retraction. Insertion of a large levator palpebrae superioris muscle into the upper eyelid further enhances opening of the palpebral fissure. Another unusual feature is the presence of tunnel-like structures that redirect the dorsal and ventral oblique muscles. The dorsal oblique muscle is redirected caudally about 90 degrees, then directed medially by another 90 degrees. These directional changes are accomplished via a connective tissue tunnel derived in part from the fibrous connective tissue of the dorsal rectus and the levator palpebrae superioris muscles. In most terrestrial mammals, a similar change in direction is accomplished by a cartilaginous trochlea. The ventral oblique muscle originates via a slender tendon from the frontal bone and undergoes a similar radical change in direction. Its tendon of insertion undergoes about a 90-degree change in direction that is accomplished through a tunnel-like structure derived from fibrous connective tissue of the ventral rectus muscle. Based on the morphology of the musculature presented, it is likely that the eyeballs and eyelids of the bowhead whale are quite mobile and appear capable of complex movement. The possibility of retraction and protrusion of the eyeball is discussed.