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Adjustment of the eye's optical power to maintain a focused image on the retina as object distance changes. For near vision the ciliary muscle contracts, tension in the zonular fibres decreases and the elastic crystalline lens becomes more rounded.

The ciliary body is the anterior continuation of the vascular middle coat of the eye. It includes the ciliary muscle and processes involved in aqueous-fluid production. Connections to the lens through zonular fibers link its muscular action to accommodation.

A contact lens is an optical device placed on the front surface of the eye. Its shape and refractive properties alter image formation, for example to correct refractive error. Unlike an implanted lens, it remains external to the internal ocular chambers.

The cornea is the transparent anterior surface of the eye and an important refracting interface. Its curvature strongly affects focus. Transparency, surface smoothness and mechanical protection must coexist in the same tissue, while the crystalline lens supplies a separately adjustable optical component.

A critical period in neural development is a bounded interval in which particular experiences have a strong role in establishing a specified function or circuit organization. Its timing must be stated for a species and a measured outcome. A sensitive period emphasizes heightened susceptibility without necessarily asserting that every later change is impossible.

A classic example comes from monocular deprivation in cats. P. B. Dews and colleagues closed one eye at different ages and for different durations, then compared vision through the two eyes when the animals were at least a year old. Starting deprivation at one month offered little protection against lasting impairment; starting at two months reduced the impairment, and starting at four months or later produced no lasting deficit detectable in their behavioural tests. The age comparison revealed a developmental window rather than merely a short-lived effect of having an eye closed.^[ref]

The design separates age at disruption, duration of disruption, and the function tested. A change in visual acuity and a change in the responses of visual cortex neurons need not have identical recovery profiles. The existence of a critical period for one visual measure does not establish a single deadline for the entire brain, or the absence of adult neuroplasticity.^[ref]

An extraocular muscle rotates the eye through its attachment to the sclera. The six globe-moving muscles have position-dependent horizontal, vertical and torsional actions.

An eye is a sensory organ that gathers optical information and converts it into neural signals. In the vertebrate eye, the cornea and crystalline lens focus light toward the retina. Image formation, sensory transduction and later visual interpretation are related but distinct stages.

An eyelid is a movable protective fold covering the exposed eye. Blinking distributes the tear film and helps clear the surface. The tarsal plate provides support, while associated muscles control opening and closure.

Researcher in developmental genetics. Halder and collaborators induced eyes at unusual body locations by directing eyeless expression in Drosophila, testing a developmental regulator through a striking gain-of-function experiment.^[ref]

The inferior oblique muscle is an extraocular muscle contributing to extorsion, elevation and abduction of the eye. Its effect depends on gaze position; elevation is especially informative when the eye is adducted. Its course differs from the rectus muscles arising posteriorly in the orbit.

The inferior rectus muscle primarily depresses the eye and also contributes to extorsion and adduction. It is supplied by the oculomotor nerve. Its vertical action is tested differently from depression supplied by the superior oblique muscle because the pulling geometry differs.

The iris is the adjustable diaphragm around the pupil. Its muscles alter the opening through which light enters the eye, while its pigment contributes to visible eye color. Changing pupil aperture and changing lens curvature are distinct optical controls.

The lateral rectus muscle abducts the eye. It is supplied by the abducens nerve, unlike most other muscles rotating the eyeball. Its relatively direct horizontal action provides a useful anatomical example connecting one muscle, one nerve and one principal movement.

The medial rectus muscle adducts the eye and is supplied by the oculomotor nerve. Coordinated activity with the opposite eye supports convergence. A local muscle action and the binocular control problem are therefore different explanatory levels.

A medical entomologist who developed methods for control of mosquitoes and eye gnats. His work connected insect population management with reduction of vector-associated health problems, combining field investigations with applied disease-prevention research.^[ref]

Monocular deprivation is an experimental reduction of normal patterned vision through one eye while the other remains available. It is used to investigate how sensory experience shapes a developing nervous system. In classic cat experiments, researchers closed the eyelids of one eye for a stated interval, then reopened it and compared what the animal could do through each eye.^[ref]

P. B. Dews and colleagues varied both the starting age and duration of deprivation, testing the cats when they were at least a year old. Closure during the first four to six weeks left a lasting reduction in visual acuity. Longer early closure could also impair visually guided paw placement and pattern discrimination. Deprivation beginning at four months or later produced no lasting deficit detectable by their behavioural tests. The comparison identifies a critical period of heightened susceptibility in this species and experimental setting; it does not mean that all forms of adult neuroplasticity disappear.^[ref]

An illuminating feature of the design is its separation of when experience is lost from how long it is lost. A weak response immediately after opening an eye would not alone demonstrate a lasting developmental change; delayed testing and comparison with the other eye make that inference stronger.

The branch of medicine concerned with the eye and visual function. Optical measurements, tissue examination and neural assessment address different sources of impaired vision.

The eye socket, distinct from a temporal fenestra behind it.

Expose the orbit (anatomy) in layers, preserving the eye, extraocular muscles, nerves and blood vessels so that their relations can be demonstrated. This is a general explanation of the practical exercise, using present-day anatomy terminology. It does not assert findings in the individual head supplied in 1867. The essential result is an intelligible dissection, with structures traced to their attachments or continuations, rather than a collection of detached parts.

Begin by orienting the head and choosing a superior approach through the anterior cranial fossa. Useful apparatus consists of a small saw or bone-cutting forceps, a fine chisel, fine scissors, dissecting forceps and a blunt probe. The coarse instruments remove bone; the fine instruments separate connective tissue. Remove the dura mater from the orbital part of the frontal bone, then carefully remove the roof of the orbit (anatomy) while leaving the periorbita intact. Extend the exposure towards the apex without driving the instrument into the optic canal or the superior orbital fissure. Preserve the delicate medial wall and its anterior ethmoidal nerve, posterior ethmoidal nerve and accompanying blood vessels. Open the periorbita under direct vision. This establishes a continuous view from the anterior margin towards the apex.^[ref]

In the superficial plane, identify the frontal nerve above the levator palpebrae superioris, and follow its division into the supraorbital nerve and supratrochlear nerve. On the lateral side identify the lacrimal gland, lacrimal nerve and accompanying lacrimal artery. On the medial side find the superior oblique muscle and the trochlear nerve entering it. Separate small amounts of adipose tissue with a blunt probe instead of stripping out the contents in a single mass. Preserve the superior ophthalmic vein where recognizable. Before reflecting any structure, show its course in place: removal of an overlying structure should answer a particular question about what lies underneath it.^[ref]

Next isolate the levator palpebrae superioris and trace it forward towards the upper eyelid. Its expanded tendon reaches the upper tarsal plate and adjacent eyelid tissues; distinguish this from an attachment to the sclera. Divide the levator palpebrae superioris across its belly and reflect the ends, keeping its oculomotor nerve supply attached where practicable. The superior rectus muscle is now exposed. Isolate and reflect it in turn, preserving the superior division of the oculomotor nerve entering the deep surface. This successive exposure demonstrates why the superficial frontal nerve should be located before either muscle is divided.^[ref][ref]

Trace the four rectus extraocular muscles from the common tendinous ring at the apex to the anterior sclera, behind the margin of the cornea. Demonstrate the medial rectus muscle and lateral rectus muscle along the corresponding sides of the eye, and the inferior rectus muscle below it. Trace the superior oblique muscle forwards to the trochlea of superior oblique; its tendon turns backwards and laterally beneath the superior rectus muscle to reach the sclera behind the equator of the eye. The inferior oblique muscle is the important exception to an apical origin: it begins on the anterior medial orbital floor of the maxilla, lateral to the lacrimal sac, and passes backwards and laterally beneath the inferior rectus muscle to the posterior lateral sclera.^[ref]

The following functional explanation makes the muscle display meaningful. The medial rectus muscle produces adduction of the eye, and the lateral rectus muscle produces abduction of the eye. The superior rectus muscle elevates and the inferior rectus muscle depresses, with additional adduction of the eye; the superior rectus muscle also produces intorsion of the eye, and the inferior rectus muscle produces extorsion of the eye. The superior oblique muscle produces intorsion of the eye, depression and abduction of the eye; the inferior oblique muscle produces extorsion of the eye, elevation and abduction of the eye. Depression by the superior oblique muscle is demonstrated most clearly with the eye adducted. Gentle traction on an isolated tendon, with the eye supported, can illustrate its line of pull in the preparation. This mechanical demonstration does not test living nerve function, and no observed movement in the historical specimen is supplied.^[ref]

In the deeper plane, identify the optic nerve entering the posterior eye and trace it back to the optic canal. Identify the nasociliary nerve crossing above it towards the medial wall; follow the long ciliary nerves towards the eye and the anterior ethmoidal nerve and posterior ethmoidal nerve towards the medial bone. On the lateral side of the optic nerve, between it and the lateral rectus muscle, seek the small ciliary ganglion. Preserve the short ciliary nerves that pass from its anterior aspect to the eye; an indiscriminate sweep through the surrounding adipose tissue would destroy precisely these small connections.^[ref]

Trace the inferior division of the oculomotor nerve towards the medial rectus muscle, inferior rectus muscle and inferior oblique muscle, including its connection to the ciliary ganglion. Trace the abducens nerve to the deep surface of the lateral rectus muscle, and the trochlear nerve to the superior oblique muscle. The lateral rectus muscle receives the abducens nerve, the superior oblique muscle receives the trochlear nerve, and the remaining listed extraocular muscles receive the oculomotor nerve. The ciliary ganglion is a relay for fibres of the parasympathetic nervous system carried with the oculomotor nerve towards the sphincter pupillae muscle and ciliary muscle. Sensory fibres connected with the nasociliary nerve, and already postganglionic fibres of the sympathetic nervous system, traverse the ciliary ganglion without another relay.^[ref]

Trace the ophthalmic artery from the optic canal and its continuity with the internal carotid artery, distinguishing it from the optic nerve. Follow recognizable branches: the central retinal artery towards and into the optic nerve, the posterior ciliary arteries towards the posterior sclera, the lacrimal artery towards the lacrimal gland, and the supraorbital artery, anterior ethmoidal artery and posterior ethmoidal artery towards their exits. The usual ophthalmic artery crosses above the optic nerve to reach the medial side, but an individual course must be established by tracing, not imposed on the preparation. ^[ref] Follow the superior ophthalmic vein towards the superior orbital fissure and cavernous sinus. The inferior ophthalmic vein also has communications towards the pterygoid plexus through the inferior orbital fissure.^[ref]

Finally, show the relations at the apex. The optic nerve and ophthalmic artery pass through the optic canal within the common tendinous ring. The divisions of the oculomotor nerve, nasociliary nerve and abducens nerve enter through the part of the superior orbital fissure enclosed by the common tendinous ring; the frontal nerve, lacrimal nerve and trochlear nerve enter outside it. Keep the eye attached until these relations have been displayed. A final labelled account should distinguish structures actually preserved from structures damaged or too small to demonstrate. That qualification matters in a practical dissection: an expected structure is a guide to search, not an observation.

Let an upward force $P$ act along $x=0$ and a downward parallel force $Q$ along $x=d>0$. An equivalent resultant force must preserve both total force and torque. Its signed vertical component is $R=P-Q$, and taking torques about the first line gives $R{x}_R=-Qd$. ThusIts magnitude is $|P-Q|$ and its direction agrees with the larger force. Its line lies outside the two original lines on the side of that larger force; for example, $P>Q$ gives $x_R<0$. The products of the original forces and their perpendicular distances from this line are equal.

If $P=Q\ne 0$ and $d\ne 0$, the total force vanishes but the torque is $-Pd$. No single force is equivalent to this system: it is a force couple of magnitude $Pd$. Its torque is independent of the reference point because translating that point changes the torque by the cross product of the shift and the zero resultant force. Equal opposite forces on the same line instead cancel completely.

The eye is rotated by six extraocular muscles. The rectus muscles approach it from behind; the oblique muscles have oblique lines of pull, with the superior oblique muscle tendon redirected by an orbital pulley. Attachments on the sclera give lever arms about the rotation centre. In the straight-ahead position their actions are as follows; vertical and torsional contributions vary with gaze direction.^[ref]

Coordinated contraction selects the desired rotation and balances unwanted components. This illustrates torque control; the full ocular geometry is not simply the parallel-force pair of the first calculation.