Biomedical Engineering Reference
In-Depth Information
Fig. 1
Distribution of rods and cones in retina
index of n
1.33. It contains a small amount of proteins providing nutrient for the
cornea and lens. The iris is a circular structure in front of the lens. Its function is to
control the diameter of the pupil and corrects optical aberration of the lens. Pupil
diameter is reflexively controlled depending on the amount of incident light on the
retina. The lens is a transparent elastic biconvex structure with a refractive index of
n
D
1.42. Its shape (and thereby focal length) is changed by contracting and relaxing
the ciliary muscle. This process is called accommodation and is driven reflexively.
The vitreous humour (n
D
1.33) fills the inner space of the vitreous chamber. Its
composition is similar to the aqueous humour and, in addition, contains vitrein
accounting for the gel-like state of the vitreous humour. The sensory part of the eye
is the retina located at the rear of the eyeball. Photoreceptors in the retina are the first
of the four neurons in the visual pathway, but they are spatially located as the last
layer of retina. There are two types of photoreceptors - cones and rods. Cones are
chromatic photoreceptors suitable for photopic vision. Their greatest concentration
is in the fovea centralis (150,000 per mm
2
), which is the point of sharpest vision.
The total number of cones is about 6-7 millions. Rods are achromatic sensors
designed for scotopic vision. The approximate number of rods is 120 million with
the greatest concentration in a circle of 5-6 mm diameter around the fovea centralis
and concentration decreases towards the periphery (Fig.
1
). More information can
be found in e.g. [
1
].
The next two neurons of the visual pathway are also located in the retina - bipolar
and ganglion cells [
1
]. The axons of the ganglion cells transmit impulses around the
inner perimeter of the eyeball and converge on the nervus opticus. Axons go through
the chiasma opticum where part of the fibres cross to the opposite nervous fascicle
(left and right tractus opticus). A small part of the tractus opticus fibres are separated
D
