Page 373 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Receptors and sense organs (organa sensuum) 355

to a dense, inner capillary network. This supplies the outer
ingly deep anterior chamber. The corneoscleral junction
VetBooks.ir is marked by an annular depression. At the periphery of retinal layers (rods and cones).
The basal complex (Bruch’s membrane) lies between
the cornea, the corneoscleral junction contains pigment
the choriocapillary layer and the outermost layer of the
deposits that can be differentiated gonioscopically into
an inner and outer pigment band (annulus corneae). retina (retinal pigment epithelium). It is relatively thick
The cornea is extremely sensitive due to the presence (1–3 μm). The basal complex contains a central layer of
of abundant sensory nerve fibres. Blood vessels – that elastic fibres inserted between layers of collagen fibres.
would compromise the transparency of the cornea – are These collagen layers lie adjacent to the capillaries of the
absent. The cornea is nourished by diffusion from the choriocapillary layer (externally) and the basal lamina of
aqueous humour and the precorneal tear film. In contrast the retinal pigment epithelium (internally). The basal com-
to mammals, birds have a thick Bowman’s membrane plex is firmly attached to the retinal pigment epithelium.
that contributes substantially to the structural integrity Species variation
of the cornea. Descemet’s membrane is relatively thin Horse, ruminants and carnivores: Dorsal to the optic
and is not present in all bird species. The corneal stroma nerve head, a crescent-shaped reflective layer, the tape-
is composed of collagen fibrils and abundant hydrophilic tum lucidum, is incorporated into the choroid, between
chondroitin sulfate-rich ground substance. Corneal the vascular and choriocapillary layers (Figures 16.21 and
transparency results from the arrangement of the colla- 16.24). In carnivores, the tapetum lucidum consists of
gen fibrils and regulation of water content in the stroma.
10–15 layers of cells (tapetum cellulosum). Herbivores
have a tapetum lucidum consisting of concentrically ori-
Vascular tunic (uvea, tunica vasculosa or ented fibres (tapetum fibrosum). In the region occupied
media bulbi) by the tapetum lucidum, the retinal pigment epithelium
The middle tunic of the bulb is a loosely arranged layer con- is non-pigmented and translucent, permitting light to
taining pigmented cells, collagen and elastic fibres, muscle, fall directly on the tapetal layer. Reflection of light from
nerve fibres and abundant vessels (Figures 16.16 and 16.17). the tapetum lucidum results in additional retinal stimu-
The vascular tunic consists of a posterior portion, the lation, improving vision in low light conditions.
choroid (choroidea), and two anterior components, the Birds: Diurnal birds have a heavily pigmented choroid,
ciliary body (corpus ciliare) and the iris. The free edge of whereas little if any pigment is present in crepuscular
the iris forms the boundary of the pupil. species (Figure 16.29). A tapetum lucidum choroideae,
present in several species of crepuscular mammals, has
CHOROID (CHOROIDEA) not been observed in birds. A white reflective area (tape-
The choroid extends posteriorly from the ciliary body at the tum lucidum retinae) is present in the dorsal fundus of
ora serrata (anterior limit of the sensory retina, see below). the European nightjar, but this is associated with the
It is extensively vascularised. Choroidal vessels are embed- retinal pigment epithelium and is not the equivalent of
ded in loose connective tissue incorporating fibrocytes, the choroidal tapetum of mammals.
melanocytes, networks of elastic fibres and lymphoid cells. Sinuses containing mucous substances are present
The layers of the choroid (Figures 16.20, 16.23, 16.24 and within the choroid of woodpeckers. These act as shock
16.29), from outermost to innermost, are the: absorbers that dampen the impact of pecking.

· suprachoroid layer (lamina suprachoroidea, lamina CILIARY BODY (CORPUS CILIARE)
fusca sclerae), The mammalian ciliary body is comprised of the following
· vascular layer (lamina vasculosa), components:
· choriocapillary layer (lamina choriocapillaris) and
· basal complex (lamina vitrea). · epithelial lining (pars ciliaris retinae),
· anterior region (pars plicata) with ciliary processes
The suprachoroid layer establishes a loose connection (processus ciliares),
between the choroid and the sclera. It consists of connec- · posterior region (pars plana, orbiculus ciliaris) and
tive tissue incorporating numerous pigmented cells. The · ciliary muscle.
vascular layer is the thickest layer of the choroid. Like
the suprachoroid layer, it is composed of lamellae of pig- The ciliary body extends anteriorly from the ora serrata
mented connective tissue. Relatively large vessels coursing (anterior limit of the choroid) to the base of the iris (margo
through the vascular layer supply the inner layers of the ciliaris iridis). It is situated at the level of the lens, to which
retina (e.g. aa. ciliares, vv. vorticosae). These vascular loops it is connected by the zonular fibres. The ciliary body forms
extend into the choriocapillary layer where they give rise the peripheral boundary of the posterior chamber of the

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