Flashcards in Glossary of terms - anatomical terms Deck (68):
Define open orbit
in most carnivores, incomplete bony orbital rim and ventral part of the orbital “cup” is made of soft tissue that is continuous with the soft tissues of the posterior oral and oropharyngeal areas, and not bone.
Define closed orbit
n herbivores and humans/non-human primates for instance. The orbital rim is complete and so is the orbital “cup”
Connective tissue that extends from the periosteum of the orbital rim to the suconjunctival tissues of the eye forming a connective tissue bridge between the two structures. It is an important surgical landmark during transpalpebral enucleation.
Which eyelid is most mobile?
Is the medial canthal ligament a true ligament?
A true, wide and very short ligament that tightly adheres the medial canthus to the orbital rim medially.
Is the lateral cathal ligament a true ligament?
Not a true ligament, more of a long, tendinous-connective tissue band.
These are lashes. There are no lashes in the lower eyelids of small animals or equines. Dogs and horses have lashes in the upper eyelids and cats have hairs that resemble lashes.
Long hairs connected to sensitive touch receptors. Horses have vibrissae around their eyes.
Another name for tarsal glands
Describe meibomian glands. What can go wrong with them?
A collection of glands that are embedded into the eyelid’s tarsal plate and stand one next to the other along the eyelid length of the upper and lower eyelid. They have an opening at the eyelid edge through which they secrete meibomium, an oily product that is spread over the tear film (see the tear film). Material can become impacted within the gland, which can burst and lead to granuloma formation around it (Chalazion) and the gland can become infected and lead to an Internal Stye (Internal Hordeolum).
What are the glands of Zeiss and Moll?
less important glands that are associated with the skin hairs and cilia and when infected can lead to an External Stye (External Hordeolum).
What is the tarsal plate/ tarsus?
A poorly developed fibrous structure of approximately 4 to 5 mm in width that runs along the upper and lower eyelid edge and gives this part of the eyelid structural rigidity. It is an important surgical landmark for several eyelid procedures and the holding layer of suturing techniques such as the figure-of-8 and tarsal plate sutures used in many eyelid reconstruction techniques. It contains the meibomian glands.
Function - orbicularis oculi mm?
strong mm around eyelids, for eyelid closure
Function - levator palpebral superioris
main mm to lift upper eyelid
Function - Müller’s muscle
Supportive muscle that helps maintain the upper eyelid up
What are the parts of the TE/ nictitans/ nictitating membrane?
- leading edge
- cartilaginous core
- gland of TE (produces approximately 30% tears)
Location - lacrimal gland
below dorsolateral orbital rim, produces about 70% tears
Parts of the conjunctiva
- conjunctival fornixes
Describe branches of conjunctival BVs
Describe the subconjunctival tissues
Tennons’s capsule and the episclera, which contains episcleral blood vessels that meander like some of the big rivers in the world but do not branch. An episcleral vessel pattern is seen typically with intraocular disease.
3 layers of tear film
Describe lipid layer of tear film
On top, secreted by the meibomian glands. Avoids evaporation of the aqueous part of the tear film.
Describe aqueous layer of tear film
Middle and most abundant layer. Contains water and many solutes and defense mechanisms that include immunoglobulins and lactoferrin.
Describe mucus layer of tear film
Innermost part. Secreted by the corneal epithelial cells and the goblet cells (mucus producing cells) of the conjunctiva.
Name 4 layers of cornea
- descemet's mebrane
Describe corneal epithelium
This is the cornea’s most external layer. A few cells thick, its basal cells in the deepest layer will ultimately be shed as squamous cells in a 1-week cycle. The basal cells at the limbus serve as stem cells that create other basal cells. A superficial corneal ulcer develops when the epithelium is missing.
Describe corneal stroma
This is the thickest part of the cornea. It is made of collagen that is laid down with a very particular fiber arrangement to allow for light to travel through them without disruption. The stroma has several layers of these fibers, much like an onion, each layer is called a lamella. A relative state of dehydration and the help of glycosaminoglycans keep the orderly state of the collagen fibers within each lamella. This, and the natural lack of blood vessels and the lack of myelin of the corneal nerves, keeps the cornea transparent.
Describe corneal descemet's membrane
During progressive ulceration of the cornea, right before perforation develops, this layer forms a descemetocele, a surgical emergency.
Describe corneal endothelium
A mere 1 cell thick layer, it keeps the cornea dehydrated by the action of NaK-ATPase pumps that take fluid that filters from the aqueous humor into the stroma, back into the aqueous humor. In practical terms it has no regenerative capabilities and a limited ability to deal with cell loss.
What is the limbus?
Trasition between the cornea and the limbus, pigmented in some animals
White fibrous tunic, continuous with the limbus and the cornea, it is covered by subconjunctival connective tissue (episclera/Tennons’s capsule) and conjunctiva. The sclera is very thin in the equator of the eye as well as near the optic disc. The sclera has several holes that allow for nerves and blood vessels to enter and exit. It also has the Lamina Cribrosa through which the axons of the ganglion cells exit the eye to form the optic nerve
What is special about the dorsal oblique extraocular mm?
(has a really cool cartilaginous pulley mechanism called the trochlea)
Describe the aqueous humor
The liquid that fills the anterior and posterior chambers of the anterior segment, it maintains IOP, it feeds the inner cornea and it is produced from blood fluids by the epithelium of the CB.
2 parts - anterior uvea
iris and CB
Describe the iris. What are the different parts of it?
It has a constrictor and a dilator muscles to open and close the pupil. The latter is the stronger of the two and during disease processes it can be contracted causing miosis. The iris has a central hole in it, the pupil. The area of the iris next to it is called the pupillary zone, the peripheral area is called the ciliary zone and the area in between is called the iris collarette.
What are persistent pupillary membranes (PPMs)?
fetal vasculature remnants that arise from the iris collarette and may stay in the iris or reach the cornea or lens, causing an opacity of either.
(adhesions of the iris to other structures) occur with inflammation of the uvea (uveitis).
Where is a posterior synechiae?
happen between the iris and lens
Where is an anterior synechiae?
happen between the iris and cornea and can only develop when there is sudden loss of aqueous humor, such as during corneal perforation.
What is the ciliary body (CB)?
Has its own mm, which is used for lens accommodation (focusing). When this musculature is contracted during disease processes it can be an important source of pain. The CB has the capacity to produce AH through various processes, one of the most important involving the carbonic anhydrase enzyme.
What is the posterior uvea?
This is the choroid. It contains the tapetum, which reflects light inside of the eye to allow for better night vision and it feeds the outer retina (mostly the photoreceptors). Carnivores have a much brighter Tapetum than herbivores, as they are made of slightly different material, and the cat probably has the brightest. Horses have the “stars of Winslow” all over their tapetal fundi. These are end-on choroidal capillaries that give the tapetal fundus a subtle, pin-prick-spotted appearance.
What is the iridocorneal angle (ICA)?
Outflow route of the AH (known as the conventional outflow route), which ultimately reaches the venous circulation. It is a 360 degree structure found between the iris and cornea (the “unconventional” or accessory outflow route is referred to as uveo-scleral outflow, as the AH is absorbed by these structures. It is a very important outflow route in the horse).
What are zonules?
These are a series of fibres connecting the CB and the lens of the eye. They arise from the ciliary body and attach to the lens, anteriorly and posteriorly to the lens equator.
Describe the lens equator
The earth’s equator exists in the horizontal axis, the lens’ equator exists in the vertical axis and it is the “edge” around the lens, it separates the anterior segment from the posterior segment.
Define anterior/ posterior lens
front or back lens’ “bellies
What is the lens axial?
an imaginary axis that passes directly through the pupil
2 parts of lens capsule
Anterior and posterior parts
Describe the lens epithelial cells and lens fibres
The former produce the latter.
Define lens suture lines
They are at the confluence of the lens fibers, in the shape of a Y in the anterior part of the lens and an upside down Y in the posterior part of the lens in dogs and cats.
Describe lens cortex
The youngest part of the lens, produced by the lens epithelial cells as they form fibers throughout the life of the lens. The fibers will loose the cell nuclei and form part of the nucleus, which is more and more compacted with age (and thus harder and a bit bluish…. This ageing change is called nuclear sclerosis).
What are the different layers of the lens nucleus?
embryonic, fetal, juveline and adult layers
Describe the layers of the retina
1 epithelial layer + 9 neural layers
Describe the retinal epithelial layer
The outer most layer, which is the RPE or retinal pigment epithelium- nurses the photoreceptors it is in contact with (see the fundus in the posterior segment).
Describe the 9 retinal neural layers
The outermost neural layers contain the photoreceptors, the rods and cones, the inner most layers contain the ganglion cells and the neural fiber layer, which are an accumulation of axons that ultimately form the optic nerve (see the sclera).
Describe retinal vasculature
In dogs and cats (and in farmed ungulates) it contains venules and arterioles that are generally paired. The former are larger. There is a dorsal one, a lateral one and a medial one. There are many smaller vessels seen branching from them. Horses have a much more subtle retinal vasculature, with only a few short BVs emanating from the optic disc only a few millimeters out into the periphery (see the posterior uvea/choroid notes on horses).
Describe optic nerve head/ disc/ papilla
It contains various degrees of fluffy myelin in the dog , which makes it look somewhat squared or even diamond shape rather than round, and it looks very round in the cat, as there is no obvious myelin in this species. Dogs have anastomotic branches of the retinal vascular on top, cats do not. In horses the optic disc is oval in the horizontal axis, it has a light hue of salmon color, and it is larger than that of dogs and cats, though it is found within a much larger fundus.
Describe optic nn
arises from the optic disc/papilla and contains fibers from the nasal (medial) and lateral retina. The fibers of the right and left optic nerve mix in the chiasm and after that point the resulting post-chiasmal nerves are called the optic tract. Both, the nerves and the tract, as well as the retinas and other neural structures, are part of the visual pathway.
2 parts of anterior segment
- Anterior chamber: Space between the cornea and the iris
- Posterior chamber: Space between the iris and lens (tiny!!!)
What is found in the posterior segment?
- vitreous body / gel
- fundus (retina/ choroid/ sclera)
- fundic reflex
What is the vitreous body/ gel?
This gelatinous structure fills the large space in the posterior segment.
Describe the fundus (retina, choroid, sclera)
This is “the back wall of the room”, where the retina is the paper on the wall, the choroid is the plaster and the sclera is the bricks that make up the external wall. The neural retinal layers are transparent and the RPE is pigmented except where the choroid has the Tapetum, so we can see it. The choroid is usually brown and it has the Tapetum and the sclera is always white. There are different degrees of pigmentation of the RPE of the retina and the choroid. Depending on the pigment combination, which can vary as much as the coats of animals, one may or may not see the choroidal BVs and/or the white sclera. Not all animals that normally have a Tapetum have one. We call these animals, which usually contain very little pigment in their eyes and coats, ATAPETAL. They will have a red fundic reflex, much like a human.
What is the fundal reflex?
Tapetal reflex and fundic reflex are used interchangeably in animals. This is the reflection of light we see coming through the pupil in response to an external light being shone into the eye. The fundic reflex is of the color of the Tapetum (usually yellow/green but it can have an orange tone). Atapetal animals have a red reflex.
Describe the anatomical path that light travels through the eye
Tear film- Cornea- anterior and posterior chambers- lens- vitreous-Retina (first inner retina, then it reaches the outer retina, where photoreceptors are). At the level of the photoreceptors the light signal is changed into a nerve signal, conducted through the visual pathway (and other pathways, such as reflex pathways for pupil movement, etc.)
What innervates the eyelid musculature?
- Orbicularis oculi muscle: CNVII (Facial)
- Müller’s muscle: Sympathetic innervation (leads to ptosis in Horner’s syndrome)
Describe the basic visual signal path once light has reached the photoreceptor layer
Rods and Cones-Inner retina-Ganglion Cells-Nerve fiber layer (axons of the ganglion cells)-Optic disc (axons go through the lamina cribrosa of the sclera and are bunched together into a nerve after it)-Optic nerve (R&L)- Optic chiasm (decussating fibers)–Optic tract (R&L)- LGN (and before it, branches for the PLR)-Optic radiation- Visual Cortex
Outline the PLR path
Rods and Cones-Inner retina-Nerve fiber layer-Optic disc (with the lamina cribrosa)-Optic nerve (R&L)- Optic chiasm (decussating fibers)–Optic tract (R&L)- area of the Pretectal nucleus, part of the E-W and anteriomedian Nucleus- Parasympathetic (PS) fibers of the Oculomotor nerve (CNIII) with Ciliary glanglion to pupillary musculature
Outline the CNs which are important in ophthalmology
- CNII: Optic nerve (vision)
- CIII: Oculomotor (dorsal, ventral and medial recti and ventral oblique) and PS input to the pupil
- CIV: Trochlear (dorsal oblique)
- CNV: Trigeminal nerve, ophthalmic branch (sensation)
- CNVI: Abducens (Lateral rectus causing abduction of the globe- lateral movement, away from the mid-line. In animals it can also pull the globe into the orbit though the abductor muscles. As this happens, the retrobulbar fat and other tissues are displaced forward and protrusion of the third eyelid results)
- CNVII: Facial nerve (blink)
- CNVIII: Vestibulocochlear (role in conjugated eye movements and ocular movement response to sound)