186 VERTEBRATE ZOOLOGY 



It is held in position by the ciliary process, to which it is attached 

 by a delicate circular ligament; these structures lie just behind 

 the iris, on the inner surface of the bony ring surrounding the 

 cornea. Cut the eyeball in two by an equatorial incision and 

 observe the inner surface. The ciliary process will be seen sur- 

 rounding the lens, and may be distinguished by its radiating fibers. 

 The ciliary muscles are also present in the ciliary process ; these 

 muscles by their contraction slightly change the shape and position 

 of the lens and thus enable it to focus upon the retina light from 

 objects at varying distances. 



Scrape away the ciliary process and note the bony ring surround- 

 ing the cornea; it is composed of a number of separate plates. 



The interior of the eye is divided by the iris and the lens into 

 two cavities: an outer one between the cornea and the iris, con- 

 taining the watery aqueous humor, and an inner one between the 

 lens and the retina, containing the jelly like vitreous humor. 



Exercise 27. Draw a diagram showing the structure of the eye so far 

 as observed. 



The Ear. The ear consists of the inner ear, or membranous 

 labyrinth ; the middle ear, or tympanic cavity ; and the external 

 auditory meatus, the passage leading from the middle ear to the 

 outside. The inner ear, to which the auditory nerve goes, is the es- 

 sential organ of hearing ; the middle ear is joined with the pharynx 

 by the Eustachian tube and is separated from the external auditory 

 meatus by the tympanic membrane, or eardrum. 



Remove the lower jaw. Note posterior to the eye the external 

 opening of the meatus. Cut away its sides on the right side of the 

 head until the tympanic membrane, which lies at the inner end of 

 the meatus, is exposed. This membrane is circular in shape and 

 semitransparent. On its inner side it is joined with the columella, 

 a rod of cartilage and bone which extends across the tympanic 

 cavity from the tympanic membrane to the inner ear. Cut around 

 the edge of this membrane, lift up an edge, and the columella 

 will be seen within. At its inner end is an oval bone called the 

 stapes, which covers an opening in the wall of the inner ear called 

 the fenestra ovalis. The columella and stapes perform the same 



