(c) The process of focusing the crystalline lens for viewing close
objects clearly is called accommodation. The process of accommodation is
accompanied by a reduction in the pupil size as well as a convergence of the two
central lines of sight (axes on bulbi oculi).
(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. Radial and circular
muscles in the iris control the size of the pupil. The radial muscles are dilators. The
circular muscles are the constrictors. By changing the size of the pupil, the iris controls
the amount of light entering the bulbus oculi.
The iris may have many different colors. Multiple genes determine
the actual color.
(6) Chambers. The space between the cornea and the lens is called the
anterior cavity. The space between the cornea and the iris is referred to as the anterior
chamber. The space between the iris and the lens is called the posterior chamber (see
Figure 5-1). 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 irioiocornealis angle.
(7) Vitreous body. Behind the lens is a jelly-like material called the vitreous
body. It fills the posterior cavity of the bulbus oculi.
5-3. THE ADNEXA
The adnexa are the various structures associated with the bulbus oculi.
a. Extrinsic Ocular Muscles. Among the adnexa are the extrinsic ocular
muscles that move the bulbus oculi within the orbit (the cavity in the upper facial skull
that contains the bulbus oculi).
b. Eyelids. Attached to the margins of the orbit, in front of the bulbus oculi, are
the upper and lower eyelids. These have muscles for opening and closing the eyelids.
The eyelashes (cilia) are special hairs of the eyelids that help protect these bulbus oculi.
The margins of the eyelids have special oil to prevent the loss of fluids from the area.
The inner lining of the eyelids is continuous with the conjunctiva, a membrane over the
anterior surface of the bulbus oculi.
c. Lacrimal Apparatus. The conjunctiva must be kept moist and clean at all
times. To do this, a lacrimal apparatus is associated with the eyelids. In the upper
outer corner of the orbit is a lacrimal gland, which secretes a lacrimal fluid (tears) into
the junction between the upper eyelid and the conjunctiva. The motion of the bulbus