The Spatial Aspect and Fine Structure of the Orbital Muscle of the Mouse

The spatial aspect of the mouse orbital muscle was studied in serial sections by light microscopy. Then the fine structure of the muscle was studied by light and electron microscopy. The orbital muscle is a large membranous smooth muscle which is the sole wall dividing the orbital cavity from the temporal and infratemporal cavities and encloses all of the orbital contents in its course. The orbital muscle arises from the upper portion of the orbital bony wall, the trochlear cartilage, the caruncle and the lacrimal canaliculi, and ends in the lower portion of these organs. The anterior margin of the orbital muscle adheres to the levator palpebrae superioris and tarsal muscles, the orbital septum and the inferior orbital margin. The orbital muscle has membranous arising and attaching tendons which contain a large amount of elastic fibers. The surface membrane of the smooth muscle cells specializes into two kinds of alternate zones; pinocytosis vescicle-rich zones and vesicle-free zones, the latter of which is attached to many myofilaments and come into close contact with the same kind of zones of another adjacent cell to form a desmosome-like junction with a minute gap of about 400A which contains only one basement lamina. Gap junctions are also often observed between the two smooth muscle cells. The mouse orbital muscle may correspond not only to the human orbital muscle and periorbita covering the inferior orbital fissure but also to the transverse ligament of Whitnall (1910) and the suspensory ligament of Lockwood (1886). The mouse orbital muscle may function in supporting the orbital contents, especially the eyeball, in regulating blood pressure in the orbital venous sinus and in checking eye movement.