The extent and com plications of the olfactory cavity are proportionate in the two Monotremes to the size of their respec tive nerves.
The optic nerve (fig. 181, 183, 2) is small in both Monotremes, in ac cordance with the dimi nutive size of the eye : the two nerves are joined by a transversely oblong chiasma.
The eye is protected, in the Ornithorhynchus, by a cartilaginous plate continued from the upper part of the orbit, which Meckel compares with the bony palpebral plates in the Crocodile. Both Monotremes have a well developed membrananic titans : there are also an upper and a lower eyelid, each of which has its proper apertor muscle.
In the Ornithorhynchus the sclerotic is carti laginous, the cornea flabby, the retina very thick : there is no trace of pecten or marsu pium : the lens is very small, two lines in vertical and transverse diameter, one line in antero-posterior diameter; the anterior surface 2 c is nearly flat, the posterior very convex. The choroid is black, without a tapetum lucidum : the pupil is circular.
The 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 thepons, 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 interruaxillary 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 hut 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 integument 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, n, 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 servea 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 cartilaginous incomplete cylinder, protected by a valve, but not provided with an external auricle.
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 and a half in length : the margins of the slit are tumid, and support a row of bristles which protect and cover the orifice when recumbent. 'File meatus is remarkably long ; the tube is strengthened in this Illonotreme by a series of incomplete cartilaginous hoops, connected to gether by a narrow longitudinal cartilaginous 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.