2_157/Histology Text Atlas_Ross_6ed2011.pdf = 902-904 The iris arises from the anterior border of the ciliary body (Fig. 24.7) and is attached to the sclera about 2 mm posterior to the corneoscleral junction. The pupil is the central aperture of this thin disc. The iris is pushed slightly forward as it changes in size in response to light intensity. It consists of highly vascularized connective tissue stroma that is covered on its posterior surface by highly pigmented cells, the posterior pigment epithelium (Fig. 24.8). The basal lamina of these cells faces the posterior chamber of the eye. The degree of pigmentation is so great that neither the nucleus nor character of the cytoplasm can be seen in the light microscope. Located beneath this layer is a layer of myoepithelial cells, the anterior pigment myoepithelium. The apical (posterior) portions of these myoepithelial cells are laden with melanin granules, which effectively obscure their boundaries with the adjacent posterior pigment epithelial cells. The basal (anterior) portions of myoepithelial cells possess processes containing contractile elements that extend radially and collectively make up the dilator pupillae muscle of the iris. The contractile processes are enclosed by a basal lamina that separates them from the adjacent stroma. Constriction of the pupil is produced by smooth muscle cells located in the stroma of the iris near the pupillary margin of the iris. These circumferentially oriented cells collectively compose the sphincter pupillae muscle. The anterior surface of the iris reveals numerous ridges and grooves that can be seen in clinical examination with the ophthalmoscope. When this surface is examined in the light microscope, it appears as a discontinuous layer of fibroblasts and melanocytes. The number of melanocytes in the stroma is responsible for variation in eye color. The function of these pigment-containing cells in the iris is to absorb light rays. If there are few melanocytes in the stroma, eye color is derived from light reflected from the pigment present in the cells of the posterior surface of the iris, giving it a blue appearance. As the amount of pigment present in the stroma increases, the color changes from blue to shades of greenish blue, gray, and, finally, brown. The sphincter pupillae is innervated by parasympathetic nerves; the dilator pupillae muscle is under sympathetic nerve control. The size of the pupil is controlled by contraction of the sphincter pupillae and dilator pupillae muscles. The process of adaptation (increasing or decreasing the size of the pupil) ensures that only the appropriate amount of light enters the eye. Two muscles are actively involved in adaptation: • The sphincter pupillae muscle, a circular band of smooth muscle cells (Plate 106, page 924), is innervated by parasympathetic nerves carried in the oculomotor nerve (cranial nerve III) and is responsible for reducing pupillary size in response to bright light. Failure of the pupil to respond when light is shined into the eye— “pupil fixed and dilated”—is an important clinical sign showing lack of nerve or brain function. • The dilator pupillae muscle is a thin sheet of radially oriented contractile processes of pigmented myoepithelial cells constituting the anterior pigment epithelium of the iris. This muscle is innervated by sympathetic nerves from the superior cervical ganglion and is responsible for increasing pupillary size in response to dim light. Just before ophthalmoscopic examination, mydriatic agents such as atropine are given as eye drops to cause dilation of the pupil. Acetylcholine (ACh) is the neurotransmitter of the parasympathetic nervous system (it innervates the sphincter pupillae muscle); the addition of atropine blocks muscarinic acetylcholine receptors, temporally blocking the action of the sphincter muscle and leaving the pupil wide open and unreactive to light originating from ophthalmoscope. The ciliary body is the thickened anterior portion of the vascular coat and is located between the iris and choroid. The ciliary body extends about 6 mm from the root of the iris posterolaterally to the ora serrata (see Fig. 24.2). As seen from behind, the lateral edge of the ora serrata bears 17 to 34 grooves or crenulations. These grooves mark the anterior limit of both the retina and the choroid. The anterior third of the ciliary body has about 75 radial ridges or ciliary processes (see Fig. 24.7). The fibers of the zonule arise from the grooves between the ciliary processes. The layers of the ciliary body are similar to those of the iris and consist of a stroma and an epithelium. The stroma is divided into two layers: • An outer layer of smooth muscle, the ciliary muscle, makes up the bulk of the ciliary body. • An inner vascular region extends into the ciliary processes. The epithelial layer covering the internal surface of the ciliary body is a direct continuation of the two layers of the retinal epithelium (see Fig. 24.1). The ciliary muscle is organized into three functional portions or groups of smooth muscle fibers. The smooth muscle of the ciliary body has its origin in the scleral spur, a ridgelike projection on the inner surface of the sclera at the corneoscleral junction. The muscle fibers spread out in several directions and are classified into three functional groups on the basis of their direction and insertion: • The meridional (or longitudinal) portion consists of the outer muscle fibers that pass posteriorly into the stroma of the choroid. These fibers function chiefly in stretching the choroid. It also may help open the iridocorneal angle and facilitate drainage of the aqueous humor. • The radial (or oblique) portion consists of deeper muscle fiber bundles that radiate in a fanlike fashion to insert in the ciliary body. Its contraction causes the lens to flatten and thus focus for distant vision. • The circular (or sphincteric) portion consists of inner muscle fiber bundles oriented in a circular pattern that forms a sphincter. It reduces the tension on the lens, causing the lens to accommodate for near vision. Examination of a histologic preparation does not clearly reveal the arrangement of the muscle fibers. Rather, the organizational grouping is based on microdissection techniques.

Таблица.  Внутренние мышцы глазного яблока = Intrinsic muscles of the eye. Перевести на русский язык = Translate into Russian c. 613. Модификация: Gray H., (1821–1865), Drake R., Vogl W., Mitchell A., Eds. Gray's Anatomy for Students. Churchill Livingstone, 2007, 1150 p. см.: Анатомия человека: Литература. Иллюстрации.
№	Muscle	Location	Innervation	Function
1	Ciliary	Muscle fibers in the ciliary body	Parasympathetics from the oculomotor nerve [III]	Constricts ciliary body, relaxes tension on lens, lens become more rounded
2	Sphincter pupillae	Circularly arranged fibers in the iris	Parasympathetics from the oculomotor nerve [III]	Constricts pupil
3	Dilator pupillae	Radially arranged fibers in the iris	Sympathetics from the superior cervical ganglion (T1)	Dilates pupil

Таблица. Эмбриональные источники развития отдельных структур глаза. Модификация: Ross M.H., Kaye G.I., Pawlina K.W., Eds. Histology: A Text and Atlas. 4th ed., Lippincott Williams & Wilkins, 2002, 864 p., см.: Гистология человека: Литература. Иллюстрации.
№	Источник	№	Производное
I	Поверхностный эктодерм	1	Хрусталик
		2	Эпителий роговицы, конъюнктива, слёзные железы и их вспомогательные структуры структуры
II	Нейроэктодерм	1	Стекловидное тело (развивается частично из нейроэктодерма чаши глазного яблока, а частично из мезенхимы)
		2	Эпителий сетчатки, радужки и ресничного тела
		3	Мышцы сфинктера зрачка и дилататора зрачка
		4	Зрительный нерв
III	Мезодерм	1	Склера
		2	Строма роговицы, ресничное тело, радужка, хороидеа
		3	Внешние мышцы глазного яблока
		4	Веки (за исключением их эпителия и конъюнктивы)
		5	Гиалоидные структуры (большая часть дегенерирует до рождения)
		6	Оболочки зрительного нерва
		7	Соединительные ткани и кровеносные сосуды глаза, костная орбита глаза, стекловидное тело

Таблица. Эмбриональные источники развития отдельных структур глаза. Модификация: Ross M.H., Kaye G.I., Pawlina K.W., Eds. Histology: A Text and Atlas. 4th ed., Lippincott Williams & Wilkins, 2002, 864 p., см.: Гистология человека: Литература. Иллюстрации.
№	Источник	№	Производное
I	Поверхностный эктодерм	1	Хрусталик
		2	Эпителий роговицы, конъюнктива, слёзные железы и их вспомогательные структуры структуры
II	Нейроэктодерм	1	Стекловидное тело (развивается частично из нейроэктодерма чаши глазного яблока, а частично из мезенхимы)
		2	Эпителий сетчатки, радужки и ресничного тела
		3	Мышцы сфинктера зрачка и дилататора зрачка
		4	Зрительный нерв
III	Мезодерм	1	Склера
		2	Строма роговицы, ресничное тело, радужка, хороидеа
		3	Внешние мышцы глазного яблока
		4	Веки (за исключением их эпителия и конъюнктивы)
		5	Гиалоидные структуры (большая часть дегенерирует до рождения)
		6	Оболочки зрительного нерва
		7	Соединительные ткани и кровеносные сосуды глаза, костная орбита глаза, стекловидное тело