(5) Iris. Another structure formed from the anterior portion of the choroid
layer is the iris. The iris is located between the lens and the cornea.
(a) The pupil is the hole in the middle of the iris. The size of the pupil is
controlled by radial and circular muscles in the iris. The radial muscles are dilators.
The circular muscles are constrictors. By changing the size of the pupil, the iris controls
the amount of light entering the eyeball.
(b) The iris may have many different colors. The actual color is deter-
mined by multiple genes.
(6) Chambers. The space between the cornea and the lens is called the
anterior cavity. The space between the cornea and the iris is called the anterior
chamber. The space between the iris and the lens is called the posterior chamber (see
fig 11-10). Both chambers of the anterior cavity are filled with a fluid called the aqueous
humor. The aqueous humor is secreted into the chambers by the ciliary body. It drains
into the encircling canal of Schlemm, located in the angle between the cornea and the
iris. This angle is called the iridiocornealis angle.
(7) Vitreous body. Behind the lens is a jellylike material called the vitreous
11-28. THE ORBIT
The orbit is the cavity in the upper facial skull which contains the eyeball and its
adnexa. The orbit is open anteriorly.
a. The floor of the orbit is generally horizontal. Its medial wall is generally
vertical and straight from front to back. Since the lateral wall and roof converge to the
rear, the orbit is a conelike cavity.
b. In the facial skull, the orbit is surrounded by a number of specific spaces.
Superiorly, the roof of the orbit is also the floor of the anterior cranial cavity, where the
frontal portion of the brain is. Just medial to the medial wall are the structures of the
nasal chamber. Inferiorly, the floor of the orbit is also the roof of the maxillary sinus.
Laterally, the wall of the orbit is the inner wall of the temporal fossa, a depression on
each side of the skull where a fan-shaped chewing muscle (temporalis M.) is attached.
11-29. THE ADNEXA
The adnexa are the various structures associated with the eyeball.
a. Extrinsic Ocular Muscles. Among the adnexa are the extrinsic ocular
muscles, which move the eyeball within the orbit. Each eyeball has associated with it
six muscles made up of striated muscle fibers.