Part 5: Orbit, Eyeball, and Associated Structures

On the demonstration specimen that has the deeper dissection completed (zygomatic arch removed), identify the region of the orbit (the conical cavity containing the eyeball) and the periorbita, which is a cone-shaped sheath of connective tissue enclosing the eyeball and its muscles, vessels, and nerves.

• On the lateral aspect of the orbit (in the demonstration specimen) observe the lacrimal gland just medial to the orbital ligament. This structure will be discussed in more detail within the Application portion of the course.

• Comment: Note the periorbita has been incised to reveal the underlying muscles of the eye, but you will now complete the dissection of the eye on an isolated cow eyeball.

Using the preserved cow eyeball at your dissection table, complete the following dissection of the eye structures specified.

Identify any remaining extrinsic muscles of the eye that may be present on your specimen. If there is muscle tissue remaining on the caudal aspect of your cow eye specimen, attempt to identify the rectus mm., retractor bulbi m., and the ventral oblique m. (See DG Fig. 5-35)

• There are four rectus muscles: dorsal rectus, ventral rectus, medial rectus, and lateral rectus. If present, these four muscles will be seen inserting on the sclera (white part) of the eyeball on the surface that corresponds with their name.

• The retractor bulbi muscle has four fascicles that surround the optic nerve at the posterior (back) of the eye. The insertions of the retractor bulbi are in between the four rectus muscles.

• There are two oblique muscles. The ventral oblique m. passes ventral to the ventral rectus muscle and is always found on the lateral aspect of the eye. The dorsal oblique will most likely not be present on the specimen, but is positioned dorsal and medial to the dorsal rectus muscle..

Identify the optic nerve after you have completed the dissection of the ocular muscles. It is the white cord-like structure attaching to the posterior aspect of the eyeball (and is quite large).

• Dissection Note: Identify the optic nerve by separating the fascicles of the retractor bulbi m. attaching to the posterior (caudal) aspect of the eyeball.

The eyeball (bulbus oculi) is composed of three concentric layers: external (outer) fibrous layer, middle vascular layer, and an inner layer called the retina. Within the cavity of the eyeball the lens, vitreous body, and aqueous humor are found. (See DG Fig. 5-38)

Using a sharp scalpel blade, make a sagittal incision through the eyeball specimen from anterior (front) to posterior (back). Begin in the middle of the cornea and extend the incision to the middle of the sclera at the back of the eye. Make another similar incision 1 cm to the lateral side of the first, but start at the same point in the cornea so that you create a wedge/pie shaped piece of the lateral wall of the eye to be reflected/removed. Identify the following structures:

• Outer Fibrous layer: The external fibrous coat covering the eye consists of the cornea and sclera. (See DG Fig. 5-38)

  • • The cornea is the anterior portion of the eye and bulges outward slightly. It is circular and transparent. The cornea refracts light through the pupil, toward the interior of the eye.

    • The sclera is the opaque white surface, covering the posterior three-fourths of the eyeball.

    • The junction between the cornea and sclera is called the limbus (aka corneoscleral junction).

• Middle Vascular layer: The middle vascular coat, also called the uvea, consists of the iris, the ciliary body and the choroid. (See DG Fig. 5-38)

  • The choroid is darkly pigmented and vascular. It lines the fundus (posterior part of the eye) and is firmly attached to the sclera. It contains the tapetum lucidum, a triangular area of yellow-green, iridescent, light-reflecting cells located dorsally within the fundus of the eye. (Be sure to look at the museum specimens in the demonstration area to observe good examples of the tapetum lucidum.)

  • The ciliary body controls the shape of the lens (ciliary smooth muscle allows the lens to become more curved during accommodation for near vision). From the ciliary body, ciliary processes and a suspensory ligament (zonules/zonular fibers) hold the lens in place.

  • The iris is the anterior portion of the vascular layer. The iris is what gives the eye its color. Smooth muscles in the iris regulate pupil size. The pupil is the opening in the center of the iris that allows light to reach the retina.

• Internal layer (Retina) : The retina, or internal coat, is the innermost layer of the eye; it contains the photoreceptor cells.

  • • Outer pigmented cells are firmly attached to the choroid. The cells nurture photoreceptor cells and absorb excess photons. The pigmented cells continue on to the ciliary body and iris, contributing pigment to these regions.

    • The optic part of the retina lines the fundus of the eyeball. The optic retina contains photoreceptors cells (rods & cones) plus bipolar neurons and ganglion cells. Axons of ganglion cells exit the eyeball at the optic disc.

      • • The optic disc is a circular white blind spot that marks the start of the optic nerve.

• Note that the fundus is the posterior lining of the interior of the eyeball. This region includes the optic disc and tapetum lucidum. It is seen with an ophthalmoscope during an eye exam.

Eyeball contents: The cavity of the eyeball contains the lens, vitreous body, and aqueous humor (fluid). Identify these structures.

• The lens is a transparent, biconvex disc attached to the ciliary body via ciliary processes and zonular fibers. In the embalmed specimens the lens will be firm and opaque (in life it is transparent and elastic).

• The vitreous body is a clear gelatinous substance located posterior to the lens, within the vitreous chamber. It serves to hold the optic retina in place against outer pigmented cells. The vitreous body occupies 80% of the volume of the eye.

• The aqueous humor is a clear, watery fluid found both anterior and posterior to the iris and pupil, within the anterior chamber and posterior chamber of the eyeball. It is secreted by the ciliary body and the fluid drains out of the eye at the base of the iris.