Biomedical Engineering Reference
In-Depth Information
Fig. 1 Illustration showing relative positioning of major orbital glands in mam-
mals. The harderian gland (HG) is located medially and behind the globe, in many
species filling a large portion of the orbit. The gland of the nictitans (GNM) is a
lacrimal gland that is located medially and a component of the base of the
nictitating membrane. The lacrimal gland (LG) is located superior-temporal and
has 5-6 ducts that empty onto the conjunctival surface. In contrast, the infra-
orbital lacrimal gland (ILG) is located ventrally in the orbit (ILG-I) or ventral
laterally extraorbitally (ILG-E)
infraorbital gland of rodents (Fig.
1
; Table
3
). Some mammals also
have the Harder's or harderian gland, which is different from the
gland of the nictitating membrane (Table
3
). It is extraglandular
tissue that extends into the orbit from the nictitans gland [
5
]. The
Harder's gland is considerably larger than the gland of the third
eyelid and is deeply seated in the orbit. The Harder's or harderian
gland contributes lipids and phospholipids to the ocular surface
[
6
-
8
]. Lipid secretions from the Harder's gland contribute to the
tear film and may have a profound effect on the pharmacokinetics of
topically applied ocular drugs. See more information on tear film
components later in this chapter.
The major structural differences between the animals regarding
eyelid anatomy and function generally revolves around the presence
or absence of a third eyelid (palpebra tertia, plica semilunaris, or
third eyelid) (Table
3
). In most animals with a third eyelid, the
action of the eyelid is generally passive and elevated only with
retraction of the globe (via retractor bulbi muscles) or when there
is decreased sympathetic tone. The presence of a third eyelid can
make examination of animals more difficult and depending on the
drug or device being evaluated, may make application of the prod-
uct more difficult. For example, the third eyelid can make place-
ment of a corneal contact lens on the ocular surface quite
challenging and may reduce retention time.
3.2.2 Eyelids/Third
Eyelid: Blink Rate
