Evidence for Active Rectus Extraocular Muscle (EOM) Pulleys in Non-Primate Mammals

Abstract

 

Purpose:

 

Orbital connective tissue structures surrounding EOMs function in humans as pulleys (P) that constrain EOM paths and influence ocular kinematics. This comparative study was conducted to investigate EOM and connective tissue structure in several lower mammalian species.

 

Methods:

 

Whole cow, dog, horse and rabbit orbits were paraffin embedded and serially sectioned at 10 µm thickness in the coronal plane. Sections were stained with Masson’s trichrome stain to identify collagen and muscle, and van Gieson’s stain for elastin. Orbital (OL) and global layers (GL) of the EOMs were identified on the basis of fiber size and color.

 

Results:

 

All species exhibited distinct OLs and GLs in all rectus EOMs, as well as connective tissue structures similar to human rectus pulleys consisting of enveloping sleeves of collagen near the globe equator in Tenon’s fascia. Dog and rabbit pulleys were more developed with higher collagen density than in cow, with more collagen on the orbital than on the global sides of the EOMs. Extensive interdigitation of OL and collagen fibers at muscle-pulley interfaces indicated OL insertions into pulleys, while GLs were contiguous with insertional tendons. Pulleys were interconnected by fibroelastic bands in all specimens. Retractor bulbi (RB) and levator (LPS) muscles had neither OLs nor pulleys.

 

Conclusions:

 

In addition to primates and rodents, the present study confirms and extends the finding of connective tissue structures corresponding to human rectus EOM pulleys in rabbit, cow, horse and dog. GLs inserted on pulleys, suggesting active control of anteroposterior pulley positions. These data support the notion that EOM pulleys are a conserved feature of mammalian orbital anatomy, and contribute to fundamental ocular kinematics.  

 

Keywords: extraocular muscles: structure