S6 OBSERVATIONS ON THE ANATOMY OF 
which extends upwards under a portion of the temporal 
bone, forming the glenoid cavity, and consequently extend- 
ing towards the zygomatic arch. The posterior cavity, 
whose parietes are in hke manner partly osseous and partly 
cartilaginous, contains the cochlear opening, and a part of 
the facial nerve in its passage outwards. A strong osseous 
plate, proceeding from the temporal bone to the base of the 
styloid process, is that which chiefly divides the anterior 
and posterior cavities from each other ; but as the parietes 
of this second cavity of the tympanum are cartilaginous on 
one aspect, viz. the lower, the foramen by which the facial 
nei^e escapes from the cranium is not completely osseous, 
but has about a third of its circle composed of cartilage. 
I shall conclude this sketch of the organ of hearing of the 
Ornithorynchus, by observing, that in no part of the ana- 
tomy of this animal have I found a greater deviation from 
the strictly mammiferous structure, as in the formation of 
the tympanic cavity, and in the small bones contained with- 
in it. 
The position of the eyes in the upper and anterior part 
of the head, close to the edge of the flap, has been already 
noticed. The eye-balls are small, and placed deep in the 
orbits ; they seemed to me quite regular, both as to inter- 
nal structure, and to the muscular and nervous parts si- 
tuated externally to the eye-balL I could not perceive very 
distinctly any lachrymal gland, though anteriorly there is 
a small duct, which would seem to lead into the cavity of 
the nose, and, if real, may be considered as analogous to 
the lachrymal duct in the Mammalia. The convexity of 
the cornea belonged to a sphere not much smaller than the 
sclerotic; the lens is flat and soft; the orbit incomplete. 
Owing to the long immersion in spirits the internal struc- 
ture of the eye-ball could not be distinctly made out, and 
