The Functional Anatomy of the Ciliary Muscle in Four Avian Species (Part 1 of 2)

The functional anatomy of avian ciliary muscles has been difficult to describe, due to the muscle's inaccessible location inside the eye. The ciliary muscle of the chicken (Gallus gallus domesticus), the pigeon (Columbia livia), the kestrel (Falco sparverius) and the hooded merganser (Margus cucullatus) (four species with differing accommodative needs) was examined histologically in both the relaxed and contracted states. The ciliary muscle of all four species can be divided into three main muscle fibre groups based on insertion and origin: anterior, posterior and internal. The anterior muscle fibre group originates at the sclera under the scleral ossicles and inserts into the inner lamellae of the cornea. During accommodation these fibres pull the cornea posteriorly, changing the curvature of the cornea. The posterior muscle fibre group originates on the sclera and inserts posteriorly onto the baseplate of the ciliary body. The posterior fibre group acts on the baseplate of the ciliary body, pulling it forward to change the curvature of the lens during accommodation. The internal muscle fibre group extends from the baseplate of the ciliary body to the inner lamellae of the cornea and thus has a role in both corneal and lenticular accommodation. Species differences do exist, as seen in subgroups determined by the orientation of the fibres between the relaxed and contracted states and the percentage of fibres within the main muscle fibre groups. In general, the majority of ciliary muscle fibres in chickens, pigeons, and kestrels are in the anterior muscle fibre group, suggesting an emphasis on corneal accommodation; in the ciliary muscle of the hooded merganser, the majority of fibres are in the internal and posterior muscle fibre groups, indicating that lenticular accommodation is the predominant form of accommodation.