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Sei Yeul Oh, Vadims Poukens, Joseph L. Demer; Quantitative Analysis of Rectus Extraocular Muscle Layers in Monkey and Humans. Invest. Ophthalmol. Vis. Sci. 2001;42(1):10-16.
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purpose. Rectus extraocular muscles (EOMs) consist of orbital (OL) and global
(GL) layers. This study enumerated the fibers in both layers along the
length of each EOM.
methods. Four human (ages 17 months–93 years) and three monkey (ages 5–7
years) orbits were serially sectioned in the coronal plane and stained
with Masson’s trichrome. All fibers of the rectus EOMs were counted
using light microscopy at midorbit in all specimens and regular
intervals throughout the orbits for one human and one monkey.
results. In the GL, human EOMs in midorbit contained 8000 to 16,400 fibers, and
monkey EOMs contained 3600 to 6600 fibers, varying little among the
four rectus EOMs. In humans and monkeys, the number of OL fibers in
midorbit varied widely according to specific EOM, being most numerous
for the medial rectus (human: 7400–14,600; monkey: 3700–7000). The
GL existed over the entire extent of each EOM from origin in the
orbital apex into continuity with the tendon inserting on the globe.
The OL was absent in the most anterior portion of each EOM, because OL
fibers inserted on the respective EOM pulley.
conclusions. Primate EOMs contain substantial numbers of OL fibers. Numerical
similarity of GL fibers is consistent with similar mechanical loading
on each of the four rectus EOMs, as required to rotate the globe.
Numerical dissimilarity of OL fibers correlates with varying mechanical
loading because of varying elasticities of connective tissues onto
which these fibers insert.
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