Anterior Eye Structures (Gelatt 5) Flashcards
What are the primary functions of blinking?
- maintains physiologic thickness of tear film
- aids movement of tears to and within nasolacrimal system
- helps eliminate particulate from the ocular surface
- protect the anterior segment from external trauma (reflex blinking)
- distributes tear film evenly over the cornea
What determines the shape of the palpebral fissure?
- medial and lateral canthal ligamentous and muscular attachments (i.e. the eyelids)
- relationship of the globe to the orbit (think large globe and shallow orbit)
(there are distinct breed differences that contribute to this)
What are some functions of the nictitans membrane?
- aids in protection of the ocular surface
- helps eliminate particulate from the ocular surface
- accessory aqueous tear production
- supports the position of the lower eyelid (occupies space in the ventromedial cup-de-sac)
- forms part of the lacrimal pool (medial canthus aspect)
What are the three eyelid reflexes?
- corneal
- palpebral
- menace
What is the difference in eyelid fusion at birth between carnivores and herbivores?
Carnivores generally fused at birth (until 10-14 days) and herbivores typically open at birth
What is the frequency of eyelid blinking in the dog?
It’s variable. Generally 3-5 times per minute, however a mean blink rate of 12.4 times per minute was deemed normal in one study (Carrington et al., 1987). Restrained animals can routinely blink 10-20 time a minute.
How often are blinks bilateral in the dog?
85% of the time
According to the Carrington study (1987), what percentage of blinks are incomplete in the dog?
66%
What do eyelids lack in the cat that are found in the dog?
Cilia
What is the rate of complete blinking in the cat?
1 to 5 complete blinks every 5 minutes (infrequent compared to the dog)
How often are blinks bilateral in the cat?
Approximately 70% of the time
How much light is transmitted through the pigmented feline eyelids when closed?
No more than 5% of light at longer wavelengths
When do kittens normally open their eyes?
Between 10-15 days after birth. (both eyes do not always open at the same time)
In the cat, how far across the cornea can the nictitans extend?
at least 2/3 of the way across
Characterize the musculature and innervation of the nictitans in the cat.
Consists of nine bands of smooth muscle lacking tight junctions between myocytes (visceral smooth muscle consists of tight junctions). The muscle does not respond to stretch stimulus. Each myocyte is innervated by one or more axons, which suggest that the control of musculature is neurogenic in nature, and is why the nictitans typically shows no spontaneous activity. The myocytes are innervated exclusively by postganglionic adrenergic sympathetic fibers who’s cell bodies are located in the anterior cervical ganglion. The axons for these nerves follow the oculomotor nerve. Although the endings of the axons are adrenergic, they do respond to acetylcholine and histamine.
