ANATOMY AND PHYSIOLOGY OF THE EYE
This
tutorial was made possible through the support of the binational US-India Fulbright
Fellowship Program, administered by the United States Educational Foundation in
India (USEFI) and the United States Information Service (USIS). The first version
of this work was created at the Postgraduate Department of Biosciences at Mangalore
University, between January and April 1998, as part of my Fulbright Fellowship
project on distance learning. I express my my sincerest thanks to the Bioseciences
Department faculty and especially to Professor M.N. Madhyastha, the Department
Chairman. Thanks are also due to the United States Education Foundation in India,
the United States Information Service, and the citizens of the United States who
provided the funding for the Fulbright Program.
It is dedicated to Tucker,
whose eyes dimmed in his old age, but whose loyalty never flagged, a dog whose
memory and example will shine forever.
Introduction
Vision is without doubt the most important of the five senses
for most vertebrates. It conveys information that is crucial to survival. Although
there are certainly situations in which other senses are perhaps important,
and there are examples of animals that have secondarily lost vision because
it isn't necessary to them, these are exceptions. Life that has evolved in and
adapted to the spectrum of solar radiation can be expected to develop a means
to use it for gathering information.
To a remarkable degree, the eyes of all vertebrates are similar
in structure and function; so similar as to be not much more than variations
on the same theme. This is due to the fact that designing an eye is, fundamentally,
a problem in applied engineering, and the architecture of such a structure is
constrained by the physics of what it has to do. The eye is a transducer,
whose function is to convert the energy of electromagnetic radiation into nervous
impulses. Its size, shape, and functions are dictated by the laws of optics
and the wavelength of the spectra to which it must respond. There aren't many
ways to do this; hence, a successful "design" that appeared early
in the phylogeny of the vertebrates has been highly conserved
throughout the entire line of vertebrate evolution, with appropriate
species differences arising as modifications to the basic pattern.
The Eye and the Camera
It's common to compare the eye to a camera, an analogy with
considerable merit. Reduced to its simplest level, a camera is a light-tight
box equipped with a lens to form an image on one side, and a means to control
illumination. The eye is built with similar provisions for focusing an image
and for controlling the input of light. The analogy can be carried further:
just as some cameras are capable of automatically focusing on a subject, and
of determining the correct exposure for ambient light conditions, so also is
the eye an autofocusing device with precise automatic exposure control. But
the eye does more than simply admit light and form an image; much of the initial
processing of the image, and a fair amount of the "decision making"
on what exactly the ultimate output to the brain is to include, is done within
the eye itself. Furthermore, a camera is capable of capturing only a single
static image (in the case of a motion picture camera, a series of them in rapid
succession); but they eye is capable of continuously processing an image and
more or less instantaneously making appropriate adjustments to its output.
This section discusses nomenclature and conventional terms
used in referring to structures in the eye.
This section provides an overview; and links to further discussion
on details on the structure and functions of the corneoscleral,
uveal, and retinal tunics.
A discussion of the embryonic formation of the eye, a key
to understanding its architecture.
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| Corneoscleral Tunic | Uveal Tunic | Retinal Tunic | Physiology of Vision | CNS Processing of Visual Signals