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note: class meeting today at 4 pm.
united community clinics at 5:30 meeting in lounge

Dr. Dodson: the adnexa of the eye (structures other than the eyeball 
itself that are associated w/the eye.

the eyeball sits in the bony ORBIT. it has a series of three wrappings 
surrouning it:

outermost PERIORBITA
middle SUPERFICIAL FASCIA
innermost DEEP FASCIA

the periorbita is considered as a sort of continuation of the periosteum. 
it's a tough piece of tissue coming off the back of the eye socket 
(apexof orbital cone) and surrounds the vessels, nerves, and muscles of 
the eye. it's rather adherent to the orbit on the dorsal and medial 
sides, and free ventral and laterally.

the [note: see dyce pp 331-335 for these topics] then the periorbita 
folds within the orbital socket and fuses to the inner surface of the 
eyelids. as it joins the eyelids there is a thickening within it, and the 
thickenings where it joins are called the superior and inferior tarsi of 
the periorbita. TARSUS actually means flat plate. 

there is a pair of contrasting anatomical terms: superior and inferior - 
very common in human anatomy, but have been eliminated from vet med 
because it's not really appropriate for the quadrupeds. however we still 
use them in the head, where they should replace the terms dorsal and 
ventral. so the palpebra superioris is the upper eyelid and the palpebra 
inferioris is the lower eyelid.

another feature in the periorbita is the trochlea (trochlea means 
pulley). the trochlea is a peculiar structure which redirects the force 
of the dorsal oblique muscle. it's a flat piece of cartilage in the 
dorsomedial area of the orbit within the periorbitum.  

the superficial fascia is thin. it surrounds the lacrimal gland and the 
levator palpebrae superioris.

the deep fascia surrounds the eyeball and its muscles, and attaches to 
the eye at the limbus, which is the junction between the cornea and the 
sclera

there are four rectus muscles and two obliques, and one retractor bulbi. 
all of these originate from the apex of the orbital cone/back wall of the 
orbit adjacent to the optic nerve. all but one focus on that single 
point. the exception is the ventral oblique muscle, which does not 
originate there but rather from the ventromedial side of the orbit. it 
courses over the ventral rectus to its insertion. the dorsal oblique 
courses over the trochlea, and its tendon of insertion goes under the 
dorsal rectus - not over it.

the rectus muscles insert on the equator of the eyeball.
forming a layer around the optic nerve inserting behind the equator of 
the eye is the retractor bulbi. very few species do not have a retractor 
bulbi - but one of those is people. in animals the eyes aren't recessed 
as much. contraction of retractor bulbi is linked with blink reflex, so 
it is a nice protective mechanism.

there is a lot of fat in the eye socket, it's a cushioning, protective 
fat pad behind/below the eye. it belongs there, and is a sign of animal 
health. starving animals often have sunken eyes because the fat pad is 
lost.

in terms of innervation...the oculomotor nerve (III) innervates 3 of the 
4 recti and 1 oblique. the DORSAL OBLIQUE is innervated by the trochlear 
nerve (IV) and the lateral rectus and the retractor bulbi are innervated 
by the abducens (VI). 

note: small animals have no lashes on the lower lid.

there is another soft tissue lining the eyelids - the conjunctiva, thin 
mucous membrane, forms a conjunctival sac. the thickenings at the free 
ends of the lids are the tarsi, and the tissue contains "tarsal glands" 
which secrete a thick lipid secretion which becomes evident when the eye 
is infected. the lipid rich stuff is what we call "sleep" in the eyes.

there is also a structure called the "third eyelid" situated at the 
medial angle of the eye, and there is an associated gland of the third 
eyelid.

[note - dr dodson is losing it. he says we have to shut up or he'll sing 
to us]

there is a T shaped cartilage associated w/the third eyelid, and both 
surfaces of the third eyelid are conjunctival.

the final thing we'll consider is the lacrimal apparatus. this is 
something to think about it. so you have the lacrimal gland...what else? 
the duct system. tears are produced by lacrimal gland, this is an aqueous 
secretion. the lipids of the tarsal gland help to distribute the tear 
layer, helping to spread them into an even film so they don't form 
discrete droplets. the conjunctiva contribute a mucinous component to the 
tears. so the tears have three parts - aqueous, lipid, mucinous. then the 
duct system which drains tears away...tears are collected at medial angle 
of eye. there are lacrimal punctae - two little holes next to the medial 
angle of the eye. there's a lacrimal sac there and a nasolacrimal duct 
beginning there, passing through a foramen in the lacrimal bone, leading 
into the maxilla, passing medially into the nasal cavity. usually it 
discharges in the nasal vestibule, in some species it's farther back.
this duct sometimes needs to be flushed out. in the large animals when we 
open the maxillary sinus we'll see the nasolacrimal duct. it might even 
show up on the practical exam.


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