386 
is nearly flat, the posterior very convex. The 
choroid is black, without a tapetum lucidum : 
a np is circular. 
nerves of the third pair (fig. 183, 3) have 
the usual origin and destination, and are like- 
wise very small; the fourth nerve is still more 
minute. 
The fifth pair in the Ornithorhynchus ex- 
ceeds, in relative magnitude, that of any other 
animal; though large, also, in the Echidna, 
its size is much less remarkable in this Mono- 
treme. 
The trigeminal nerve in the Ornithorhynchus, 
(fig. 181, 5,) emerging from the ganglion an- 
terior to the pons, soon divides into three 
branches ; the first and second appearing as one. 
The first and smallest division divides into two 
equal branches: the superior or ethmoidal 
branch enters the nose, emerges from a canal 
in the upper part of that cavity, and supplies 
the skin at the upper part of the face; and, 
by a branch continued from between the nasal 
and intermaxillary bones, is distributed to the 
nostrils and contiguous integument. 
The second division of the fifth is two lines 
broad and one line and a half thick; it passes 
through the foramen rotundum, and the chief 
part of it passes into the ant-orbital canal. 
On its emergence it divides into two branches, 
distributed, the one to the nasal or upper pa- 
rietes of the face, the other to the lateral or 
labial integuments. The palatine branch di- 
vides into a posterior smaller nerve, which 
passes through the posterior palatine foramina : 
the anterior and larger branch emerges from 
the anterior palatine canal and supplies Jacob- 
son’s organ and the surrounding palatine mem- 
brane. 
Thethird division of the fifth (5’) is broader but 
thinner than the second; it leaves the cranium 
by the foramen ovale, and is distributed as 
usual, in part to the manducatory muscles, 
but mainly to the sensitive labial mtegument 
of the lower jaw (fig. 180, a a). 
The sixth nerve (fig. 183, 6) is as small as 
the third. The seventh and the acoustic pre- 
sent half a line in diameter. 
The acoustic nerve is expended upon a 
labyrinth remarkable for the small relative size 
of the semicircular canals, and their free pro- 
jection into the cavity of the cranium. 
The cochlea is wide, but not high; it is bent 
around a modiolus, and divided as usual into 
a superior and inferior scala. 
The foramen ovale is nearly circular, and 
opens into the wide but shallow tympanic 
cavity. It is naturally closed by the base of a 
small columelliform and imperforate stapes 
(fig. 173, p, d): the stem of this ossicle is 
articulated with a triangular plate of bone (c), 
representing, according to Meckel, the incus. 
This bone is connected with a small bent os- 
seous style (b), which serves to complete, with 
the similarly-shaped tympanic ossicle (a), the 
frame supporting the membrana tympani. 
This membrane is concave externally, and 
forms the inner extremity of a long and narrow 
meatus auditorius externus, which is strength- 
ened by a cartilagmous incomplete cylinder, 
MONOTREMATA. 
protected by a valve, but not provided with 
an external auricle. ‘4 
The auricle is equally wanting in the 
Echidna, in which the external aperture of the 
auditory canal presents the form of a vertical 
slit, shaped like the italic S, one inch anda 
half in length: the margins of the slit are 
tumid, and support a row of bristles which 
rotect and cover the orifice when recumbent. 
he meatus is remarkably long; the tube is — 
strengthened in this Monotreme by a series of 
incomplete cartilaginous hoops, connected to= —— 
gether by a aarrow longitudinal ilagi a. 
band, so that its structure closely resembles 
that of a trachea (fig. 188, a, a). The tym- 
panic fossa is almost entirely encircled with a 
slender hoop of bone (fig. 169, c) consisting of 
the anchylosed tympanic bone and malleus. 
The portion which represents the tympanic 
bone (a), and which can be separated from the 
malleus in the youngsubject, is a slender osseous 
filament bent into three-fourths of a cirele, 
and placed upon the inner margin of the tym- __ 
panic fossa, its concavity looking outwards: — 
this concavity is impressed with a fine groove 
ms the insertion of the membrana tympani : 
the posterior of the hoop passes across 
the eine of the Restneliiln 
and terminates in a free point upon the pos- 
terior wall of the tympanic fossa: the or 
end of the hoop is applied to and usually 
anchylosed with the longitudinal bar of the — 
malleus (b). ' 
Only a small portion of this ossicle is con- 
an 
tained within the cavity of the tympanum ; the 
principal portion forms the external and part 
of the posterior boundary of the bony meatus 
satneny and is then continued forwards in 
the form of a slender pointed process; 
bone slightly expands as Ly extends backwards, 
and its broadest part is abruptly bent inwards 
until it nearly meets the posterior end of the 
tympanic hoop. From the extremity of this in- 
fected portion a slender compressed red 
2 
w 
extends to the centre of the space encircled by — 
the bony hoop; it is attached by its whole 
length to the membrana tympani, and repre- 
sents the handle of the malleus, At the pos- 
terior margin of the broad incurved part of the 
malleus there are two minute tubercles 
a line apart: the short and simple columelliform 
stapes (d) ascends vertically from the inner- — 
most of these tubercles, with the upper surface 
of which it is articulated ; its ite ex- 
tremity closes the foramen ovale im the form — 
of an expanded plate. The membranetyai | 
pani is concave outwardly at its middle part. — 
The eighth and ninth pairs of nerves — 
have the usual origins and proportions. The 
pneumogastric nerve (fig. 180, d) is 
attached, at its origin, to the hy ' 
(fg. 180, 6), but is quite distinct from the © 
sympathetic (fig. 180, f): it gives off the 
superior laryngeal, and then long 
the neck to the chest: the right nerve here 
sends its recurrent branch, in the usual manner, 
round the arteria innominata; the left branch 
(fig. 187, k) winds round the aorta: the trunk 
of the pneumogastric is then expended in the 
‘ 
