Lab 22 Pre-Lab Exercise:
1. Bony landmarks of the orbit
The orbits are bilateral structures in the upper half of the face below the anterior cranial fossa and anterior to the middle cranial fossa that contain the eyeball, the optic nerve, the extra-ocular muscles, the lacrimal apparatus, adipose tissue, fascia, and the nerves and vessels that supply these structures. Seven bones contribute to the framework of each orbit: maxilla, zygomatic, frontal, ethmoid, lacrimal, sphenoid, and palatine.
Roof: superior wall of the bony orbit is made up of the orbital part of the frontal bone. The supraorbital margin, where the supraorbital nerve (CN V1) exits, can be palpated.
Medial rim: of the paired bony orbits are parallel to each other and consists of the end of the orbital frontal bone, maxilla, lacrimal, and ethmoid bones. The lacrimal and maxilla bones participate in the formation of the lacrimal groove, which contains the lacrimal sac. This wall is also the location of the fronto-maxillary suture.
Lateral rim: composed by the end of the orbital frontal bone (zygomatic process of the frontal bone) and the zygomatic bone (frontal process of the zygomatic bone). The lateral border is more open making this a common approach to surgery. Palpate the lateral wall of your orbit. This wall is also the location of the zygomatico-frontal suture.
Floor: inferior wall of the bony orbit (which is also the roof of the maxillary sinus) consists of the orbital surface of the maxilla as well as the zygomatic bone. The infraorbital margin, where the infraorbital nerve (CN V2) exits, can be palpated.
2. Soft tissues of the orbit
Major features of the eye include the sclera, cornea, iris and pupil. The cornea is continuous with the sclera and it is the clear circular region of the external covering of the eye through which the pupil and iris are visible. The sclera is not transparent, and is normally white.
Palpebral fissure: is the interval between the upper and the lower lids. When the eye is closed it lies just below the level of the pupil. Eyelashes are absent at the medial margins of the palpebral fissure. The eyelids come together at the medial and lateral palpebral commissures (ligaments) on either side of the eye.
Palpebral commissures/ligaments: fibrous bands stretching the junction of the lids on either side of the corresponding margin of the orbit. In thin people, they may raise up a fold of skin especially on the medial side. They are known as the medial and lateral palpebral ligaments.
Upper eyelid: much bigger than the lower and its deep surface, it is best seen by turning the lid inside out. This may be achieved by everting the lid over a rod or pencil held horizontally over the lid, above the level of the eyeball, and exerting upward and outward traction on the eyelashes. In this case the superior fornix of the conjunctiva is still not seen even when the eye is rotated downward.
Tarsal glands: visible through the conjunctiva when the upper lid is everted. They appear as parallel yellow streaks at right angles to the palpebral fissure. Their oily secretion prevents the overflow of tears and the eyelids from sticking together.
Tarsal plate: makes the upper lid rigid and becomes obvious when the upper lid is everted.
Lacrimal lake is a small triangular soft tissue structure at the medial side of the palpebral fissure and lateral to the medial palpebral commissure. The lacrimal caruncle is an elevated mound of pink tissue on the medial side of the lacrimal lake. The lacrimal fold is found on the lateral margin overlying the sclera. The lacrimal papilla is a small elevation seen at the medial end of each lid where the eyelashes stop. It is more easily seen on the lower lid.
Conjunctiva: covers the eyeball and lines the deep surface of the eyelids to form the conjunctival sac (thin gap between the deep surface of the eyelid and the cornea), which is sealed when the eyelids are shut. The blood vessels of the ocular conjunctiva are so small that it appears almost clear, but the palpebral conjunctiva is more vascular and appears red or pink. The palpebral conjunctiva of the lower lid is readily visible and provides a useful guide to the level of hemoglobin in the blood (in anemic patients, its appears pale).
Plica semilunaris (semi-lunar folds): small fold of conjunctiva immediately lateral to the lacrimal caruncle. It represents the vestigial nictitating membrane (this translucent membrane in other animals moves horizontally across the eye and can protect the eye while underwater, or from dust and debris).
The lacrimal apparatus consists of the lacrimal gland and the system of ducts and channels that collect the ears and drain them into the nasal cavity. Tears hydrate and maintain the transparency if the cornea.
Lacrimal gland: associated with the upper eyelid and is lodged in a fossa behind the supero-lateral part of the orbital margin under cover of the zygomatic process of the frontal bone. It is divided into a large orbital portion and a smaller palpebral portion by the lateral part of the aponeurosis of the levator palpebrae superioris muscle. The multiple small ducts of the gland open into the upper margin of the conjunctival sac.
Tears are swept medially over the eye by blinking (rapid relaxation of levator palpebrae superioris and contraction of orbicularis oculi) and are collected in small opening (punctum lacrimale), one on each of the upper and lower eyelids near the lacrimal lake. Each punctum is on a small raised mound of tissue (lacrimal papilla) and is the opening of a small canal (lacrimal canaliculi) that connects with the lacrimal sac. Each canaliculus has a vertical and a horizontal portion that is approximately 10 mm long. When the eye blinks, the canaliculi are shorten and compressed by otbicularis oculi, pumping the tears into the lacrimal sac.
Lacrimal sac: lies behind the palpebral ligament and sits in the lacrimal fossa and drains into the nasolacimal duct. The infratrochlear nerve (CN V1) passes above the sac.
Nasolacrimal duct: passes downwards and backwards from the sac in the lateral wall of the nose for about 18 mm before opening into the inferior meatus of the nose.
3. External ear
The external ear consists of the auricle and the external acoustic meatus (EAM). The auricle is supported by cartilage and is covered by skin. It is characterized by a number of depressions, eminences, and folds. The folded outer margin of the auricle is the helix, which ends inferiorly as the lobule. A smaller fold (the antihelix) parallels the contour of the helix and is separated from it by a depression (the scaphoid fossa).
The external auditory meatus is near the anterior margin of the auricle. Its outer 1/3 is cartilaginous and the inner 2/3 lie in the petrous temporal bone. The tragus is a small eminence anteroinferior to the EAM. Opposite the tragus and at the end of the antihelix is another eminence (the antitragus). The depression between the tragus and the antitragus is the intertragic incisure. The deepest depression (the concha) is bracketed by the antihelix and leads into the EAM. Other depressions include triangular fossa and the cymba conchae.