THE AGCESSOBY ORGANS OF VISION 831 



well as a row of erect hairs, the eyelashes: these will be described 

 presently. 



When the two lids are closed by the approximation of their free borders, 

 they completely cover the eye, and form a narrow fissure comparable to a 

 closed button-hole. When they are separated, they circumscribe an oval 

 space {fissura palpebrarum), whose greater axis is directed obliquely down- 

 wards, forwards, and inwards. The upper lip or contour of this space, 

 formed by the free margin of the superior eyelid, is always more curved 

 than the lower. The superior commissure (or canthus) has also been named 

 the temporal angle of the eye. The nasal angle, constituted by the inferior 

 commissure, is always rounder than the other j it lodges the lachrymal 

 caruncle (in the lachus lachrymalis). 



STRncTUEE or THE Etelids. — A fibrous plate, terminated, towards the 

 free border of the lid, by a small tendinous arch named the tarsus; a 

 sphincter muscle, the orbicularis palpebrce, in contact with the fibrous mem- 

 brane ; the levator palpebrce, a muscle partly lodged in the ocular sheath, 

 and terminated anteriorly by a very thin and wide expansion placed beneath 

 the superior fibrous plate ; a cutaneous envelope in two parts, containing 

 the above : an external, the skin ; and an internal of mucous membrane, the 

 conjunctiva, joining at the free border of the lid ; — these are the elements 

 which enter into the composition of the protective coverings of the eye. 



1. PiBROUs Membrane. — Usually thicker in the lower than the 

 upper lid, this membrane is attached, by its adherent border, to the rim of 

 the orbit. Where it is continuous with the periosteum and the fibrous wall of 

 the ocular sheath. Its free border is margined by the tarsus. 



2. Tarsus. — This is a fibrous lamella that forms a solid frame for the 

 free border of the lid : it is elongated, narrow at its extremities, thin at its 

 fixed border, where it is confounded with the fibrous membrane, and chan- 

 neled on its inner face by several transverse parallel grooves which lodge 

 the Meibomian glands. This small fibrous arc regulates the contraction of 

 the orbicularis muscle, and prevents the lid being drawn into wrinkles ; by 

 the rigidity it bestows on the eyelids, it allows these to meet, border to 

 border, without puckering, when that muscle is in action. 



3. Orbicular Muscle of the Eyelids (musculus ciliaris). — This is a 

 wide thin sphincter common to the two lids, applied to the fibrous mem- 

 brane and the bone forming the rim of the orbit. Its external face is 

 covered by the skin, to which it closely adheres. A small tendon that 

 passes to the lachrymal tubercle of the nasal angle of the eye, is generally 

 considered as the origin of the fibres of this muscle, the majority of which, 

 directed upwards, are disposed in a circular manner in the substance of 

 the upper lid ; while the others go to the lower lid, both joining at the 

 temporal angle of the eye. 



The contraction of this muscle causes the occlusion of the palpebral open- 

 ing. (It is a prominent agent in defending the eye from external injury.) 

 We may regard as an appendage of the orbicularis, a little short, flat fascicu- 

 lus usually designated the fronto-superciliary muscle from its attachments (or 

 corrugator supercilii, from its function). It arises from the outer face of 

 the frontal bone, passes downwards and outwards, and mixes its fibres with 

 those of the latter muscle at the super orbital foramen, which it covers, and 

 in the skin of the eyebrow. It has been erroneously considered an elevator 

 of the upper lid, for when it contracts, it only corrugates the skin of the 

 eyebrow by slightly drawing the nasal angle of the eye outwards ; this it 

 does as well when the lids are closed as when they are open. 



