Comparative anatomy of the circular muscle layers in mammalian orbit and their possible functions

• Main Authors: Keiko Meshida, Helen Wimer, Erica Miller, Edwin Gilland
• Other Authors: Surgery, SOM
• Format: Other/Unknown Material
• Language: unknown
• Published: Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814 2023
• Subjects: Pharyngeal Muscles; Sperm whale
• Online Access: http://cdm16005.contentdm.oclc.org/cdm/ref/collection/p16005coll8/id/449

Introduction: This study further examines the circular muscle layers that were recently found on both the external and internal surfaces of the extraocular muscles (EOMs) in cetaceans (Meshida et al., 2020). An External Circular Muscle (ECM) formed a cone-shaped, multi-layered smooth muscle sheet in all cetaceans examined except in a juvenile sperm whale (USNM 594183 right), whose ECM comprised mixed layers of smooth and striated muscle. The cetaceans also possessed striated sheets of an Internal Circular Muscle (ICM) located between the internal surface of the rectus EOMs and a connective tissue layer external to the retractor bulbi muscle (RB). A similar ICM has not been described in non-cetacean species. The present study examines 1) presence of striated muscle in ECM/orbitalis muscle in other mammals, 2) presence/absence of ICM in non-cetacean mammals, 3) possible functions of ECM and ICM. Introduction: This study further examines the circular muscle layers that were recently found on both the external and internal surfaces of the extraocular muscles (EOMs) in cetaceans (Meshida et al., 2020). An External Circular Muscle (ECM) formed a cone-shaped, multi-layered smooth muscle sheet in all cetaceans examined except in a juvenile sperm whale (USNM 594183 right), whose ECM comprised mixed layers of smooth and striated muscle. The cetaceans also possessed striated sheets of an Internal Circular Muscle (ICM) located between the internal surface of the rectus EOMs and a connective tissue layer external to the retractor bulbi muscle (RB). A similar ICM has not been described in non-cetacean species. The present study examines 1) presence of striated muscle in ECM/orbitalis muscle in other mammals, 2) presence/absence of ICM in non-cetacean mammals, 3) possible functions of ECM and ICM. Summary • ECM/orbitalis smooth muscle is generally present in mammals and is functionally suited for several roles depending on orbital and ocular configuration (e.g., ocular protrusion, ventrolateral support of orbital contents, ...