Eyelids

00:01 Now we come to the last topic of our discussion, the accessories structures of the visual apparatus.

00:07 The first of this structures we will be discussing are the eyelids.

00:12 The eyelids are modified skin that protect the eyeball from injury, shield it from excessive light and prevent it from getting dry by spreading tears over the cornea and then facilitating tear outflow through the nasal lacrimal apparatus.

00:28 So each eye has to eyelids or palpebrae.

00:31 The upper and lower, the upper and lower eyelids are separated by a transverse opening called the palpebral fissure and are joined at the medial lateral ends of the fissure and these are the medial and lateral canthus.

00:44 The medial canthus has several anatomic landmarks located around it.

00:49 Between the medial canthus in the eyeball, there's a small triangular space called the lacrimal lake which contains a small reddish elevation called the lacrimal caruncle which is considered a vestigial structure.

01:03 Additionally, medial and lateral to each canthus on the margins of the eyelid are located elevated lacrimal papillae, which contain a small aperture called the lacrimal punctum that allows the drainage of tears into the nasal lacrimal apparatus.

01:22 Furthermore, these papillae are used to divide the eyelid margin lengthwise into parts.

01:28 Lateral to each papilla is located the cilliary part of the eyelid which contains the eyelashes immediately it was located the lacrimal part of the eyelid which is devoid of lashes.

01:40 Since we have just mentioned the eyelid margin, I'd like to mention that this margin is also divided internal posteriorly or along its width.

01:48 The posterior 1/3 or around one millimetre the eyelid margin is mucosal tissue containing the tarsal meibomian glands and the anterior 2/3 are around two millimetres in the eyelid margin is cutaneous containing the eyelashes.

02:06 This mucocutaneous separation between the anterior and posterior borders of the eye lash margin is demarcated by a Grey line.

02:15 Now I'd like to talk about the structure of the eyelid is made up of the following structures from anterior to posterior.

02:22 The skin of the eyelid is composed of keratinized stratified squamous epithelium, and is the thinnest in the body.

02:29 This skin is continuous with adjacent facial skin without obvious demarcation.

02:36 Underneath the skin lies a sparse amount of subcutaneous connective tissue that contains the eyelash follicles and follicle associated sebaceous glands of Zeiss and modified apocrine select lands a moll.

02:53 Then comes the fibres of the palpebral parts the orbicularis oculi, and another thin layer a submuscular connective tissue lies underneath the orbicularis oculi.

03:06 Then we have a thick layer that's formed by the tarsal plates, and the tarsal meibomian glands.

03:13 Tarsal plates are thickened interior extensions of the orbital septum and are composed of dance connective tissue.

03:21 They are secured to the margins of the orbit by the orbital septum, which we discussed earlier, and by the medial and lateral palpebral ligaments.

03:31 The tarsal meibomian glands are modified sebaceous glands which secrete oily hydrophobic substance, which reduces the evaporation of the protective tear film and prevents the spillage of tears over the eyelid it onto the face.

03:49 Lying underneath the tarsal plates in the case of the upper eyelid is the aponeurosis of the levator palpebrae superioris muscle and deep to this is located the palpebral conjunctiva.

04:02 I would like to briefly mention what the conjunctiva is, the conjunctiva is a highly vascular mucous membrane that binds the inside of the eyelids, and then at the fornix reflects onto the sclera until the point of its junction with the cornea.

04:17 And atomically, the regions of the conjunctiva are divided into the marginal tarsal, orbital, fornix and bulbar, and limbal sections.

04:28 A point worth mentioning is that the conjunctiva has varied an interesting histologic makeup which dictates its function.

04:36 However, this is beyond the scope of our discussion today.