{"id":3065,"date":"2020-12-13T21:28:09","date_gmt":"2020-12-13T19:28:09","guid":{"rendered":"https:\/\/meddists.com\/learn\/pre-clinical\/histology\/neurohistology\/eyes-eyelids\/middle-layer\/"},"modified":"2021-12-04T12:03:20","modified_gmt":"2021-12-04T10:03:20","slug":"middle-layer","status":"publish","type":"page","link":"https:\/\/meddists.com\/learn\/pre-clinical\/histology\/nervous-system\/eyes-eyelids\/middle-layer\/","title":{"rendered":"Middle layer"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><div class=\"intro\">The middle layer of the eye consists of the\u00a0iris,\u00a0ciliary body, and\u00a0choroid.<\/div><\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"768\" src=\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/external-eye-1024x768.png\" alt=\"\" class=\"wp-image-8746\" srcset=\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/external-eye-1024x768.png 1024w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/external-eye-300x225.png 300w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/external-eye-768x576.png 768w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/external-eye.png 1186w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption><strong>Figure 1. Section of the eye showing the external and middle portions of the eye.<\/strong><\/figcaption><\/figure>\n\n\n<span class=\"block-heading\" id=\"header_1\">\n<h3 class=\"wp-block-heading\" class=\"wp-block-heading\" class=\"title_collection title1\">Iris<\/h3>\n<\/span><span class=\"block-content\" id=\"contents_1\">\n\n\n<p class=\"wp-block-paragraph\">Arising from the ciliary body and the sclera, the iris forms the contractile diaphragm of the eye and controls how much light enters the pupil. It is comparable to a camera&#8217;s shutter.<\/p>\n\n\n\n<div class=\"wp-block-columns is-layout-flex wp-container-core-columns-is-layout-8f761849 wp-block-columns-is-layout-flex\">\n<div class=\"wp-block-column is-layout-flow wp-block-column-is-layout-flow\" style=\"flex-basis:66.66%\">\n<p class=\"wp-block-paragraph\">The iris is highly vascularized, consisting of three structures:<\/p>\n\n\n\n<ul class=\"wp-block-list\" id=\"block-f0c14e8d-9b3b-442d-b402-3d528d7e8660\"><li><strong>Posterior pigmented layer<\/strong><ul><li>Highly pigmented epithelium<\/li><\/ul><\/li><li><strong>Dilator pupillae muscle<\/strong><ul><li>Specialized contractile epithelium &#8212; <strong>not<\/strong> smooth muscle cells, but rather\u00a0<strong>myoepithelial cells<\/strong> with contractile processes<\/li><li>Innervated by unmyelinated sympathetic fibers from the superior cervical sympathetic ganglion<\/li><\/ul><\/li><li><strong>Sphincter pupillae muscle<\/strong><ul><li>Circumferentially oriented smooth muscle<\/li><li>Innervated by parasympathetic nerve fibers derived from the oculomotor nerve, which comes from the ciliary ganglion via the short ciliary nerves<\/li><\/ul><\/li><\/ul>\n<\/div>\n\n\n\n<div class=\"wp-block-column is-vertically-aligned-center is-layout-flow wp-block-column-is-layout-flow\" style=\"flex-basis:33.33%\">\n<figure class=\"wp-block-image size-medium\"><a href=\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology.png\" target=\"_blank\" title=\"Middle layer\"><img loading=\"lazy\" decoding=\"async\" width=\"300\" height=\"151\" src=\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology-300x151.png\" alt=\"\" class=\"wp-image-8769\" srcset=\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology-300x151.png 300w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology-1024x515.png 1024w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology-768x386.png 768w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology-1536x773.png 1536w, https:\/\/meddists.com\/learn\/wp-content\/uploads\/2021\/12\/iris-histology.png 1749w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption><strong>Figure 2. Histologic slide of the iris<\/strong><\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n<\/span><span class=\"block-heading\" id=\"header_2\">\n<h4 class=\"wp-block-heading\" class=\"wp-block-heading\" class=\"title_collection title2\">Pupil<\/h4>\n<\/span><span class=\"block-content\" id=\"contents_2\">\n\n\n<p class=\"wp-block-paragraph\">The pupil is the central aperture (hole) which allows light to enter the eye and changes its size according to the amount of ambient light thanks to the function of the iris.<\/p>\n\n\n<\/span><span class=\"block-heading\" id=\"header_3\">\n<h3 class=\"wp-block-heading\" class=\"wp-block-heading\" class=\"title_collection title1\">Ciliary body<\/h3>\n<\/span><span class=\"block-content\" id=\"contents_3\">\n\n\n<p class=\"wp-block-paragraph\">The ciliary body is the anterior part of the vascular coat between the iris and choroid.<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>The ciliary body layers are somewhat similar to those of the iris, containing stroma and epithelium<\/li><li>The inner stroma of the ciliary body contains an&nbsp;<strong>inner vascular region<\/strong> which is full of capillaries, and the&nbsp;bulk of the ciliary body stroma consists of the outer&nbsp;<strong>ciliary muscle<\/strong>, a smooth muscle that has three orientations &#8211;&nbsp;<strong>meridional<\/strong> (longitudinal) for stretching the choroid,&nbsp;<strong>radial<\/strong> (oblique) for flattening of the lens, and&nbsp;<strong>circular<\/strong> for the relaxation of the lens.<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Zonular fibers<\/strong> emerge from the\u00a0<strong>ciliary processes<\/strong> (extensions of the\u00a0ciliary body) and insert onto the lens, anchoring it in its place, and serving for changing its shape during accommodation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The ciliary body and its processes are covered by\u00a0<strong>ciliary epithelium<\/strong>, two layers of columnar epithelium that have three functions:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Production of aqueous humor<\/li><li>Participation in the&nbsp;blood-aqueous barrier (which is formed mainly by tight junctions of the blood vessel&#8217;s endothelium)<\/li><li>Production and anchoring of zonular fibers<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">The anterior part of this epithelium (the one facing the iris) is pigmented, while the inner part (the one facing the eye cavity) is non-pigmented. The internal surface epithelium is a direct continuation of the retinal epithelium.<\/p>\n\n\n<\/span><span class=\"block-heading\" id=\"header_4\">\n<h3 class=\"wp-block-heading\" class=\"wp-block-heading\" class=\"title_collection title1\">Choroid<\/h3>\n<\/span><span class=\"block-content\" id=\"contents_4\">\n\n\n<p class=\"wp-block-paragraph\">The choroid is the brown, vascular coating layer that is located between the sclera and the retina until the margin of the sclera. It consists of two layers:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Bruch&#8217;s membrane<ul><li>Thin, outer layer\u00a0consisting of mainly elastic and collagen fibers<\/li><li>Located between the choriocapillary layer and the retina<\/li><li>Also called <strong>lamina vitrea<\/strong> due to its reflectiveness<\/li><\/ul><\/li><li>Choriocapillary layer<ul><li>An inner layer of fenestrated capillaries arranged in a single plane, which provide nutrition to the retina<\/li><li>This layer gets denser at the <strong>fovea<\/strong>, except for the\u00a0<strong>foveal avascular zone<\/strong>, which is clear of blood vessels.<\/li><li>Bordered by the\u00a0<strong>ora serrata<\/strong><\/li><\/ul><\/li><\/ul>\n\n\n<\/span><span class=\"block-heading\" id=\"header_5\">\n<h3 class=\"wp-block-heading\" class=\"wp-block-heading\" class=\"title_collection title1\">References<\/h3>\n<\/span><span class=\"block-content\" id=\"contents_5\">\n\n\n<ol class=\"wp-block-list\"><li><strong>Figure 1.<\/strong> Blausen.com staff (2014). CC BY-SA 4.0<\/li><li><strong>Figure 2. <\/strong>P. Stanka, CC BY-SA 4.0<\/li><\/ol>\n<\/span><div id=\"the_titles\" style=\"display:none;\"><h3 class=\"wp-block-heading\" class=\"wp-block-heading\">Iris<\/h3><h4 class=\"wp-block-heading\" class=\"wp-block-heading\">Pupil<\/h4><h3 class=\"wp-block-heading\" class=\"wp-block-heading\">Ciliary body<\/h3><h3 class=\"wp-block-heading\" class=\"wp-block-heading\">Choroid<\/h3><h3 class=\"wp-block-heading\" class=\"wp-block-heading\">References<\/h3><\/div>","protected":false},"excerpt":{"rendered":"<p>Iris Arising from the ciliary body and the sclera, the iris forms the contractile diaphragm of the eye and controls how much light enters the pupil. It is comparable to a camera&#8217;s shutter. The iris is highly vascularized, consisting of three structures: Posterior pigmented layer Highly pigmented epithelium Dilator pupillae muscle Specialized contractile epithelium &#8212; [&hellip;]<\/p>\n","protected":false},"author":23,"featured_media":0,"parent":3033,"menu_order":2,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-3065","page","type-page","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Middle layer &#8211; Meddists<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/meddists.com\/learn\/pre-clinical\/histology\/nervous-system\/eyes-eyelids\/middle-layer\/\" \/>\n<meta name=\"twitter:label1\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data1\" content=\"3 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/meddists.com\\\/learn\\\/pre-clinical\\\/histology\\\/nervous-system\\\/eyes-eyelids\\\/middle-layer\\\/\",\"url\":\"https:\\\/\\\/meddists.com\\\/learn\\\/pre-clinical\\\/histology\\\/nervous-system\\\/eyes-eyelids\\\/middle-layer\\\/\",\"name\":\"Middle layer &#8211; 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