WILTIMS #54: Eye spy

Today we explored the orbit (socket) of the eye, both in lecture and in lab. The eye is incredible both as a complex sensory organ and as a clinical tool. The movement of the eye is controlled by six muscles which are innervated by three distinct cranial nerves. This apparent inefficiency is a great diagnostic tool for classifying nerve or brain damage and the reason for those seemingly bizarre "follow my finger" eye tests.

In lab, we skinned the eyelids and explored the tear glands and ducts. We then turned to the space behind the eye, which was only accessible by breaking through the roof of the orbit with a chisel. The complexity of the eye was finally exposed after nearly an hour of meticulously clearing out all of the fat that cushions the eye and associated muscles, nerves and blood vessels.

TIL: The superior rectus muscle of the eye is innervated by the same nerve as the superior levator palpebrae, the upshot of which is that you can't look up without pulling the upper eyelids up as well.

The superior oblique muscle (the topmost muscle in the accompanying diagram) actually uses a naturally formed pulley system. In order to exert more force on the eye, the muscle passes from its attachment on the eye through a tendinous loop called a trochlea (literally: pulley wheel), before finally travelly to the back of the eye socket.

Tarsal glands are located on the edges of the eyelids and secrete an oil that both keeps the eyelids from sticking together as you blink and acts as a hydrophobic barrier, keeping the tears within the eye.

The lacrimal caruncle (literally: teary meaty lump) on the nasal side of the eye surrounds a duct which siphons excess tear fluid into the nasal cavity. Ever notice that when a person gets misty eyed, they start sniffling? Turns out the bulk of  fluid that ends up in their tissue is tears, not snot. The tears are secreted from the outer upper part of the eye socket, flow over the surface of the eye, are held within the socket by the oil on the eyelids, collect near the meaty lumps and drain into the inferior nasal meatus of the nose.

During embryonic development we form remnants of gill slits that we inherited from our common ancestor with the jawless lamprey eel. The tissue between these slits eventual develop into the jaw, hyoid bone, and the cartilaginous structures of the pharynx and trachea. Bony fish evolved a covering over their gill slits and, sure enough, embryologically you can see the analogous human structure that forms from this covering: the external structures of the ear.