Structure of the muscles of the upper eyelid
T. Kuwabara, D. G. Cogan and C. C. Johnson
The human and monkey orbicularis muscle has fibers that are more uniform in
size and structure than those of rectus muscles. They have distinct
myofibrils, a moderate number of mitochondria, and a well-developed
transverse T-tube system. The levator muscle also has relatively uniform
fibers, but the myofibrils are less distinct than those of the orbicularis.
Especially noteworthy is the unusual arrangement whereby Muller muscle
arises directly from the undersurface of the levator muscle, causing an
intimate intermingling of smooth and striated fibers. Muller muscle then
inserts on the tarsus, whereas the levator muscle extends by an aponeurosis
into the septa of the orbicularis muscle. In surgical specimens from
patients with ptosis, the levator fibers show varying degrees of
abnormality, whereas Muller fibers are normal.
