Page 374 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

356 Veterinary Histology of Domestic Mammals and Birds

eye and is in contact with the vitreous body (Figures 16.13 prevents peripheral illumination of the sclera, which is
VetBooks.ir and 16.16). It is covered externally by the sclera. incompletely covered by the orbit. The zonular fibres
The surface of the ciliary body is lined with a double
arise from the basal lamina of the inner epithelial layer
layer of epithelium, the pars ciliaris retinae, consisting
of an inner non-pigmented layer and an outer pigmented and pass to the lens capsule. Their role in attaching the
ciliary body to the lens is considerably less significant
layer. The pars ciliaris retinae, together with the pars irid- than in mammals (Figure 16.14).
ica retinae (see below) constitutes the non-sensory portion In contrast to mammals, the ciliary muscle is striated.
of the retina (pars caeca retinae) (Figure 16.11). It is embedded within the stroma of the ciliary body.
The anterior portion of the ciliary body (pars plicata, The ciliary muscle consists of two parts: the m. ciliaris
corona radialis), has numerous meridional folds (70–80 anterior and the m. ciliaris posterior. Both muscles arise
in the dog, over 100 in the ox and horse). These extend from the sclera deep to the scleral ring. The m. ciliaris
towards the lens equator as the ciliary processes (proces- anterior passes in an anterior direction to attach to the
sus ciliares). The core of the ciliary processes comprises corneal stroma. The posterior muscle extends posteri-
loose connective tissue and dense capillary networks. orly to join the base of the ciliary body (Figure 16.14).
Aqueous humour is secreted by the ciliary processes into The main action of the m. ciliaris anterior is to
the posterior chamber (Figure 16.13). alter the curvature of the cornea (corneal accom-
In the posterior region of the ciliary body (pars plana, modation) (Figure 16.14). Contraction of the m.
orbiculus ciliaris) the meridional folds are reduced in ciliaris posterior draws the ring-shaped ciliary body
height. The zonular fibres that suspend the lens originate forward, thus also reducing its diameter as the scle-
from ciliary epithelium of the pars plicata and pars plana. ral ring narrows anteriorly. Constriction of the ciliary
The connective tissue stroma of the ciliary body con- body alters the shape of the lens, increasing its con-
tains elastic fibres, melanocytes, vessels and the smooth vexity (lenticular accommodation) (Figure 16.14).
ciliary muscle. The m. ciliaris is responsible for accommo- In nocturnal birds, such as owls, accommodation is
dation. It is relatively weak in the horse, and comparatively predominantly corneal. The m. ciliaris anterior is well
well developed in carnivores. developed in these birds, while the m. ciliaris posterior
In mammals, the ciliary muscle consists of circular, is a rudimentary structure. In diving birds, which rely
meridional and radial components intermingled with elastic almost exclusively on lenticular accommodation, the
fibres. Contraction of the various components of the mus- relative degree of muscle development is reversed.
cle alters the tension in the zonular fibres, thus changing The cilioscleral sinus (sinus cilioscleralis) divides
the shape of the lens. An increase in convexity of the lens, the ciliary body into inner and outer sections. The two
particularly on its anterior surface, facilitates near vision; sections are connected only at the posterior aspect of
flattening of the lens permits focusing for distant vision. The the pars plana. At the level of the iris base, the cilio-
ciliary muscle receives parasympathetic (from the ciliary scleral sinus communicates with the anterior chamber
ganglion) and sympathetic innervation via the ciliary nerves.
at the iridocorneal angle (angulus iridocornealis). The
Species variation cilioscleral sinus is traversed by a mesh of delicate con-
nective tissue fibres (reticulum trabeculare), enclosing
Birds: The ciliary body is anchored externally to the
osseous scleral ring (annulus ossicularis sclerae). As the spaces of Fontana (spatia anguli iridocornealis)
in mammals, the anterior portion of the ciliary body is (Figure 16.14). The transition to the iridocorneal angle is
covered in numerous meridionally oriented folds (pars demarcated by strong, straight fibre bundles that anchor
plicata, corona radialis) that taper off posteriorly to the base of the iris to the sclera (ligamentum pecti-
form the pars plana (pars plana, orbiculus ciliaris). natum) (Figure 16.14). The cilioscleral sinus permits
In contrast to mammals, the ciliary processes attach to the inner section of the ciliary body and its associated
the periphery of the lens, their tips fusing with the lens folds to become displaced, thus serving to underpin the
capsule (Figure 16.14). The surface of the ciliary body is avian mechanism of lenticular accommodation. For this
lined by the double-layered pars ciliaris retinae (outer reason, the cilioscleral sinus of diving birds (in which
layer of pigmented cells, inner layer of columnar epi- extreme compression of the lens results in a particularly
thelial cells; Figure 16.27). wide range of accommodation) is relatively deep.
In most bird species, the inner layer of the pars cili-
aris retinae is non-pigmented. Nocturnal owls are an IRIS
exception. In this species, the cells of the inner layer The iris is the most anterior portion of the vascular layer.
contain large granules filled with lipofuscin (Figure It projects into the interior of the eye from the base of
16.28). This additional pigment aids in the absorption the ciliary body, partially covering the lens. The circular
of scattered light by the long, tubular ciliary body and free edge of the iris (margo pupillaris) surrounds a central

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