The Crystalline Lens (h).—The lens is a biconvex transparent body situated within the eye a little in front of the ciliary body and just behind the iris. It is more strongly convex in front than behind. The lens is surrounded by a thin transparent sac, the capsule of the lens (capsula lentis). The capsule of the lens is attached all around the equator of the lens by fibrous bands to the ciliary body, this attachment forming the zonula ciliaris (or zonula Zinni) (i), by which the lens is suspended.
Chambers of the Eye.—The lens (h) and zonula ciliaris (i) form a partition dividing the eye into two parts. The cavity in front of the lens contains a fluid, the aqueous humor. This cavity is partly subdivided by the iris into the anterior and posterior chambers of the eye.
The part of the eye lying behind the lens is much larger than the part in front of the lens. It is lined chiefly by the retina, its front boundary being the lens. It contains a jelly-like substance, the vitreous humor (or vitreous body, corpus vitreum).
II. THE EAR. ORGANON AUDITUS.
The ear of the cat, like that of man, is composed of three parts, the external, middle, and internal ear.
The external ear is limited internally by the tympanic membrane.
The middle ear extends from the tympanic membrane to the fenestra cochleæ and fenestra vestibuli. It contains the bones of the ear, and is connected with the pharynx by the Eustachian tube.
The internal ear is wholly contained within the petrous bone, and on the surface of the membrane lining its cavities are distributed the branches of the auditory (eighth) nerve.
The External Ear.
The external ear consists of an expanded portion, the auricle (auricula) or pinna, and of a canal leading from the auricle to the tympanic membrane, known as the external auditory meatus. The concave surface of the auricle is directed craniolaterad, and its edges form the dorsal and lateral boundaries of the auditory opening. From the bottom of the cavity which it partly encloses, the external auditory meatus passes medioventrad.