Page 14 - LN-NEURAL CONTROL & COORDINATION

Parts of an Eye

The adult human eye ball is nearly spherical in structure. It consists of tissues
present in three concentric layers
(i) Outermost fibrous layer composed of sclera and cornea.
(ii) Middle layer consists of choroid, ciliary body and iris.
(iii) Innermost layer consists df retina.
Outermost Layer
(i) Sclera is an opaque outermost covering, composed of dense connective
tissue that maintains the shape of the eyeball and protects all the inner layers

of the eye.
(ii) Cornea is a thin transparent, front part of sclera, which lacks blood vessels
but is rich in nerve endings.
Middle Layer
(i) Choroid is a pigmented layer (bluish) present beneath the sclera. It contains
numerous blood vessels and nourishes the retina. The choroid layer is thin
over the posterior two-thirds of the eye ball, but it becomes, thick in the

anterior part to form the ciliary body.
(ii) The eye ball contains a transparent crystalline structure called lens. Ciliary
body holds the lens in position, stretching and relaxation of ciliary body
changes the focal length of the lens for accomodation.
(iii) Iris forms a pigmented circle of muscular diaphragm attached to the ciliary
body in front of the lens. Its pigment gives eye its colour.
The movement of muscle fibres of iris controls the size (diameter) of pupil.
(iv) Pupil is the aperture surrounded by the iris. It contains two types of

smooth muscles, circular muscles (sphincters) and radial muscles (dilators) of
ectodermal origin.
(v) Sympathetic stimulation causes the radial muscles to contract and the pupil
to dilate or get larger. Parasympathetic stimulation causes the circular muscles
to contract and the pupil to constrict.
Inner Layer

The inner layer is the retina and it contains three layers of cells from inside to
outside, i.e., ganglion cells, bipolar cells and photoreceptor cells.