@article{12472717a1f449b0898af66e0c0a1c49,
title = "Cetacean Orbital Muscles: Anatomy and Function of the Circular Layers",
abstract = "Dissections of cetacean orbits identified two distinct circular muscle layers that are uniquely more elaborate than the orbitalis muscles described in numerous mammals. The circular orbital muscles in cetaceans form layers that lie both external and internal to the rectus extra ocular muscles (EOMs). A cone-shaped external circular muscle (ECM) that invests the external surface of the rectus EOMs was found in all cetacean specimens examined. The cetacean ECM corresponds generally to descriptions of the musculus orbitalis in various mammals but is more strongly developed and has more layers than in noncetaceans. A newly identified internal circular muscle (ICM) is located internal to the rectus EOMs and external to the retractor bulbi (RB). The RB is massive in cetaceans and is encased in a connective tissue layer containing convoluted bundles of blood vessels. The most robust ECM and ICM layers were in sperm whale (Physeter macrocephalus) where they form complete rings. Surprisingly, histological analysis showed the sperm whale ECM to contain both smooth and striated (skeletal) muscle layers while the ICM appeared to contain solely skeletal muscle fibers. The extreme development of the ECM (orbitalis) and RB suggest a co-evolved system mediating high degrees of protrusion and retraction in cetaceans. We know of no homolog of the ICM but its function seems likely related to the complex vascular structures surrounding and deep to the retractor muscle. Skeletal muscle components in orbital circular muscles appear to be highly derived specializations unknown outside of cetaceans. Anat Rec, 2019.",
keywords = "cetacean, circular muscle layer, orbit, orbitalis",
author = "Keiko Meshida and Stephen Lin and Domning, {Daryl P.} and Reidenberg, {Joy S.} and Paul Wang and Edwin Gilland",
note = "Funding Information: The authors thank the Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution for providing the access to their specimens. The authors also thank the following institutions for the original acquisitions of the cetacean specimens: Hawaii Sea Life Park; International Fund for Animal Welfare, Marine Mammal and Rescue and Research; Marine Mammal Stranding Center, NJ; Maryland Department of Natural Resources; National Aquarium Baltimore; National Marine Fisheries Services-Hawaii, Northeast Fisheries Science Center and Southeast Fisheries Science Center; New England Aquarium; Virginia Institute of Marine Science and Nature Conservancy; Virginia Aquarium and Marine Science Center. The authors appreciate the assistance from Mr. John Ososky, Mr. Darrin Lunde for acquisition and loaning the specimens and Ms. Helen Wimer for preparing the histological samples. The authors also thank Dr. Erica Miller at the University of Pennsylvania for donating terrestrial mammal specimens. We thank Dr. Martine deWit at the Florida Fish and Wildlife Conservation Commission and Dr. Jim Valade and Dr. Lisa Lierheimer at the U.S. Fish and Wildlife Service for providing a manatee specimen for the project. The authors also thank Dr. Gary Wind at the Uniformed Services University of the Health Sciences for the illustration (Fig. 3). This project was supported in part by the National Institute on Minority Health and Health Disparities of the National Institutes of Health (NIH/NIMHHD) under Award Number G12MD007597 and National Science Foundation (NSF) under Award Number 0009328. Publisher Copyright: {\textcopyright} 2019 American Association for Anatomy",
year = "2020",
month = jul,
day = "1",
doi = "10.1002/ar.24278",
language = "English",
volume = "303",
pages = "1792--1811",
journal = "Anatomical Record",
issn = "1932-8486",
number = "7",
}