Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 66

The eyelids of domestic animals protect the eye, particularly the cornea. All domestic animal species have a superior (upper) and an inferior (lower) eyelid; most have a nictitating membrane (NM, third eyelid). The eyelids contain the meibomian glands; these are large sebaceous glands that secrete the outer, oily layer of the precorneal tear film (PTF). The conjunctiva lines the inside of the eyelids and reflects onto the globe contains goblet cells that contribute the mucin to the PTF; accessory lacrimal glands are also present in some species. The normal blinking of the eyelids maintains the physiological thickness of the preocular tear film, aids movement of the tears both to and within the nasolacrimal system, and helps eliminate small particles from the corneal and conjunctival surfaces. Reflex closure of the eyelids protects the anterior segment from external trauma.

The eyelids determine the shape and width of the palpebral fissure, along with the associated medial and lateral canthal ligamentous and muscle attachments. For example, a wide, round palpebral fissure is normal among brachycephalic breeds, and a narrow, almond‐shaped palpebral fissure is normal among dolichocephalic breeds. The shape of the palpebral fissure also depends on the relationship of the globe to the orbit. A small globe in a deep orbit allows a narrow palpebral fissure; the opposite occurs with a large globe in a shallow orbit. The NM aids in protection of the conjunctiva and cornea by moving, either passively or actively, over the cornea when the globe is retracted.

Eyelid closure is mediated by the efferent fibers of the facial nerve (CN VII) and their effects on the orbicularis oculi muscles. The oculomotor nerve (CN III) innervates the levator palpebral superioris, which is responsible for opening the upper eyelids. Eyelid closure is the end result of two eyelid reflexes, the corneal and palpebral reflexes, and the menace response (Table 2.1). The corneal and palpebral reflexes are primitive reflexes with a purely subcortical course. Both are elicited by touch, with the afferent pathway being the ophthalmic branch of the trigeminal nerve (corneal) or the ophthalmic and maxillary branches of the trigeminal nerve (palpebral). The efferent pathway of these two reflexes as well as the menace response is the facial nerve stimulating the orbicularis oculi muscles, resulting in a blink. These reflexes are present immediately following birth or eyelid opening. In contrast, the menace response is cortically mediated and is initiated by a threatening gesture or loud sounds.

Table 2.1 Reflexes involving the blink response.

	Cornea reflex	Palpebral reflex	Menace responsea	Dazzle reflex
Stimulus	Corneal touch	Eyelid touch	Menacing gesture	Bright light
Receptors	Somesthetic	Somesthetic	Photoreceptors	Photoreceptors
Afferent pathway	Trigeminal nerve (ophthalmic)	Trigeminal nerve (ophthalmic and maxillary)	Optic nerve	Optic nerve
Interneuron	Subcortical	Subcortical	Cortical, cerebellum	Subcortical
Efferent pathway	Facial nerve	Facial nerve	Facial nerve, VI, IX	Facial nerve
Effectors	Orbicularis oculi muscle	Orbicularis oculi muscle	Orbicularis oculi muscle, retractor bulbi muscle	Orbicularis oculi muscle
Response	Blink	Blink	Blink, retract globeb	Blink

^a If sufficient cortex of one cerebral hemisphere is damaged, the menace reaction cannot be elicited in the contralateral eye of the dog. Pathology of the cerebellar cortex can also affect the menace reaction.

^b The menace response can also involve turning the head or moving away from the stimulus.

Table 2.2 Blinking rates of domestic animals.

Species	Blinks/min	Interblink period	Concurrent blinks (%)
Dog	3–5	20–30 sa	85
Cat	1–5 per 5 min	18.5 s	70
Horse	19		77
Cattle	5		60
Pigs	10		90

^a Dogs have partial blinks every few seconds between complete blinks.

Blinking and blink rates have been studied in many species under varying circumstances and methodologies, making comparisons and generalized statements difficult (Table 2.2). Blinking does not occur randomly, and blinks are often associated with gaze shifts and saccades. One of the theories for this timing is that blinking temporarily blocks visual information, and blinking during gaze shifts and saccades takes advantage of blocking vision when the images are already degraded from movement. Diurnal nonhuman primates and birds have higher blink rates than nocturnal nonhuman primates and birds, and in general, larger mammals and primates blink more often than smaller mammals. Blink rates and evaporation of the PTF can affect topical drug activity.