Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 46

The ciliary body muscle is comprised of smooth muscle fibers in mammals. Contraction of the ciliary body muscle draws the ciliary processes and body both forward and inward, thus relaxing the lenticular zonules (suspensory ligament of the lens) and altering the shape and refraction of the lens. This muscle is often weakly developed in many nonprimate species and, as a result, offers poor accommodative ability. On the basis of ciliary body musculature development, the placental mammalian ICA has been categorized into three main groups, the herbivorous, the carnivorous, and the anthropoid, and seems based primarily on the lens rather than aqueous humor outflow (Figure 1.38).

The herbivorous type has been characterized as the most common and primitive in orders of mammals up to and including ungulates. This type of angle consists of an inner layer of connective tissue that forms a baseplate of the ciliary body and extends from the root of the iris to the ora ciliaris retinae. It also consists of an outer layer of smooth muscle that presses against the sclera externally and runs meridionally from the corneoscleral junction toward the ora ciliaris retinae. The two layers are often referred to as “leaves” that separate anteriorly forming the ciliary cleft. The ciliary cleft is then a triangular area that varies in both depth (i.e., length) and height, and functionally may be considered a posterolateral extension of the anterior chamber into the ciliary body. Historically, this region was initially called the cilioscleral sinus, but the term cilioscleral sinus has been replaced with ciliary cleft. The ciliary cleft is an area containing wide spaces filled with aqueous humor and interspersed with cell‐lined cords of connective tissue. The spaces between the fibrous cords were initially described in cattle and horses, and they have been often referred to as Fontana's spaces.

Figure 1.38 Degree of development of the ciliary body musculature among mammalian ICAs in the ungulate (a), carnivore (b), and ape (c). The ciliary body musculature is most pronounced in primates and least developed in ungulates. The size of the ICA and its cilioscleral cleft or sinus (CC) is inversely large or most pronounced in the ungulate.

The carnivorous type possesses a bi‐leaflet configuration as well, but the fibrous inner leaf or layer is usually replaced by meridionally oriented smooth muscle and some radially oriented muscle fibers. In both the herbivorous and carnivorous types, the ciliary cleft offers little support to properly anchor the iris. Compensation for wide and deep ciliary clefts is provided by a series of pectinate ligaments attaching the anterior iridal root and inner ciliary baseplate to the limbal cornea.

The ciliary body musculature of primates is believed to be the most highly developed among mammals. The muscle, which has three components (i.e., radial, meridional, and circular), forms a large, anterior pyramidal structure that provides a strong baseplate for iridal attachment. The anterior portion of the ciliary body muscle has replaced both the ciliary cleft, which barely exists in the anthropoid angle, and the pectinate ligaments, which vestigially consist of scattered iridal processes in primates, including humans.

In birds and other nonmammalian species, the ciliary body muscle consists of skeletal muscle cells that are primarily meridional. At least two distinct muscle bundles are located in this region of the avian eye: an anterior bundle, which is known as the muscle of Crampton, arises near the corneal margin; and a posterior bundle, which is known as Brücke's muscle. Contraction of Brücke's muscle causes the ciliary body to push against or compress the lens, thus deforming it, while contraction of Crampton's muscle alters the shape of the cornea by shortening its radius of curvature.