EYELIDS. 695 
Chambers of the Eye and Aqueous Humour (lig. 495).—As already stated 
(p. 687), the space between the cornea and the lens is divided by the iris into two 
unequal parts, viz. the anterior chamber in front and the posterior chamber behind. 
These are filled by the aqueous humour, and, in the adult, com- 
municate freely through the aperture of the pupil, but in the 
foetus are separated from each other by the pupillary mem- 
brane. The anterior chamber (camera oculi anterior) 1s 
bounded in front by the cornea, behind by the iris and lens, 
whilst peripherally it communicates with the spaces of 
Fontana. The posterior chamber (camera oculi posterior) is 
triangular on section, and is bounded in front by the iris, 
behind by the circumferential part of the lens and its 
suspensory ligament; the base of the triangle, situated 
externally, corresponds with the thick, anterior extremities of 
the ciliary processes. It communicates with the recessus 
camer posterior and canal of Petit. The aqueous humour 
has a refractive index of about 1556 and consists of about 98 
per cent. of water, with 1:4 per cent. of sodium chloride and 
traces of albumen. 
EYELIDS. 
The eyelids or palpebrze are two movable cutaneous 
curtains situated in front of the eyeball, and named, from 
their position, upper and lower. The upper is the larger and 
more movable, being provided with a special elevator muscle, 
the m. levator palpebrze superioris. The interval between the 
lids is termed the palpebral fissure (rima palpebrarum), and 
measures transversely about 50 mm., but varies considerably in 
different individuals and in different races. When the eye 
is open the fissure is elliptical in shape, but when closed it 
assumes the form of a transverse slit, which hes on a level 
with the lower margin of the cornea. The two lids meet at | ; 
; We bain ie je : : SECTION THROUGH ‘THE 
the extremities of the fissure, and form the outer and inner © pocgsror ov rHE Lens. 
angles, or canthi. Their free margins are flattened and are ghowing the gradual tran- 
surmounted by eyelashes from the external canthus to a _ sition of the epithelium 
point about 5mm. from the inner canthus—a point indicated ae fibres (eiieer 
by asmall papilla, the papilla lacrimalis. Internal to this papilla wo 
the margins are rounded and destitute of eyelashes, and form the upper and lower 
boundaries of a triangular space, termed the lacus lacrimalis, which is occupied by 
a small pale-red body, the caruncula lacrimalis. ‘This caruncula consists of a minute 
island of modified skin, and contains sweat glands, sebaceous glands, and fine hairs. 
Posteriorly, the lds are lined by mucous membrane and are in contact with the 
eyeball, except near the inner canthus, where, between the eyeball and the carun- 
cula lacrimalis, there intervenes a vertical fold of conjunctiva, the plica semilunaris 
conjunctive. This, which in many animals contains a plate of cartilage, is the 
representative of the membrana nictitans, or third eyelid of birds, ete. 
In each lid there exists a framework of condensed fibrous tissue, which gives 
consistence and shape to the lid, and is termed the tarsal plate or tarsus. In front 
of the tarsus are the fibres of the orbicularis palpebrarum muscle and the 
integument, while embedded in its posterior surface, and covered by the conjunctiva, 
are numerous moditied sebaceous glands, named the tarsal or Meibomian glands. 
The superior tarsal plate (tarsus superior) is larger than the inferior and of a halt 
oval shape, with its greatest vertical diameter measuring about 10 or 11 mm. Its 
upper margin is thin and convex, and is continuous with the tendon of the levator 
palpebrze superioris muscle, while its lower edge is thick and straight. The inferior 
tarsal plate (tarsus inferior) is a thin, narrow strip, with a nearly uniform vertical 
diameter of about 5 mm. The extremities of the two plates are continuous 
with the external and internal tarsal ligaments. The external tarsal ligament is a 
narrow band attached to the malar bone; it divides, at the outer canthus, into 
ia. 506. 
