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Sarah Miller COPLOW Intern University of Wisconsin http://www.boatus.com/cruising/ithaka/200612-01.asp Features of Mammalian Eyes • • • • • • • No bone or cartilage in sclera No skeletal muscle No photomechanical movement in RPE No fovea Most have blood vessels within the retina (Holangiotic) Accommodation limited by passive action of lens capsule on lens –No accommodation via lens capsule Dichromatic vision- cetaceans monochrome – L-opsin (green) only Evolution • • • Artiodactyls (even-toed ungulates) Closest relative thought to be hippo Similar features – Fibrous Tapetum lucidum – indented blood vessels-whales not indented http://www.sheppardsoftware.com/content/ani mals/animals/mammals/animal_of_the_week/a nimaloftheweek_whalehippo.htm http://3rdarm.biz/images/2006/05/thehippobeluga2.JPG Fibrous Tapetum Lucidum Hippo (H&E) Dolphin (trichrome) The Difficulty of Underwater Vision • Refractive Index • Large changes in light • Heat loss • High Pressure Difficulty of Underwater Vision-R.I. • Refractive Index air =1 – R.I. terrestrial cornea=1.35 • Refractive index water =1.35 – R.I. marine cornea =1.37 Cornea doesn’t provide effective refraction! Built for Underwater Vision-R.I. Cornea of varying thickness ÆÆ • Spherical lens • Heterogeneous lens Round lensÆ – Lower R.I. in periphery • Cornea – Divergent lens • R.I. 1.37 (central) 1.53 (periphery) Built for Underwater Vision-Light • covers 2/3 to entire fundus Built for Underwater Vision-Light • Modified iris (Operculum) -Protects from large changes in illumination Operculum Iris muscle Built for Underwater Vision-Temp • Fatty eyelids • Highly vascular plexus around optic nerve – Heater – Oxygen Built for Underwater Vision-Temp Built for Underwater Vision-Pressure • Thick Sclera…why? – Pressure and thermal protection? • No adaptation needed – Fluid continuum of body compensates • Another suggestion – Protect ON from high IOP Terrestrial Vision should be myopic due to R.I. of air but not… http://media.canada.com/9ef63c09-385e-4b40bac2-da66c75d8b27/mdf41628.jpg http://img2.travelblog.org/Photos/4797/34565/t/188488-Dolphins--TrainerHoop-Trick-0.jpg http://upload.wikimedia.org/wikipedia/commons/c/c2/Peeking_Orca.jpg Shaped for success Flattened anterior segment 150° hemisphere Not good enough! Theory 1- Axial Displacement • Backward movement of lens • T. truncates seen to move eye 10-15 mm forward • Decreased IOP flattens cornea Theory 2- “spoon-shaped” divots • Depressed regions near nasal and temporal margins • Nasal and temporal regions of increased ganglion cell density *Line up via center of lens* Theory 3- Heterogeneous lens • Use different parts of lens with different R.I. – Underwater Æ center of lens – Air Æ periphery Additional Unique Features… http://images.inmagine.com/img/designpics/dpic039/dp1766255.jpg Movement • Horizontal, vertical, and axial movement • Independent eye movement – horiz and vert – Pupil reflexes From Zhu et al Anatomical Record (2000) Staying Focused • Accomodation? • Ciliary muscles underdeveloped or absent Dog Whale Staying Focused • Large protractor/retractor muscles • IOP increases when eye is pulled back, shifting lens forward • Lens moves backwards when eye moves forward Complex nerve endings • Found in sclerocorneal trabecular meshwork • Pressure reception ciliary body stroma Bowman’s Layer Very large ganglion cells • Can get larger than 75 um – Average 20-30 um • Terrestrial large 15-30 um • Y-cells (movement) Very large ganglion cells Whale Dog References • Ya Supin, Alexander, et al. The Sensory Physiology of Aquatic • • • • • • Mammals. Boston, Kluwer Academic Publishers, 2001. (pp 239264) Mass A. & Ya Supin A. (2007) Adaptive Features of Aquatic Mammals’ Eye. The Anatomical Record 290:701-715. Ninomiya H. & Yoshida E. (2007) Functional Anatomy of the Ocular Circulatory System: Vascular Corrosion Casts of the Cetacean Eye. Veterinary Ophthalmology 10(4): 231-238. Zhu Q. et al. (2000) Observations on the Muscles of the Eye of the Bowhead Whale, Balaena mysticetus. The Anatomical Record 259:189-204. Peichl L. et al. (2001) For Whales and Seals the Ocean is Not Blue: A Visual Pigment Loss in Marine Mammals. European Journal of Neuroscience 13: 1520-1528. Kroger RHH & Kirschfiels K. (1992) Optics of the Harbor Porpoise Eye in Water. J. Opt. Soc. Am. A 10:1481-1489. Vrabec F. (1972) Encapsulated Sensory Corpuscles in the Sclerocorneal Boundary tissues of the Killer Whale Orcinus orca L. Acta Anat. 81: 23-29.