286 



MONOTREMATA. 



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 trigeininal 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. 



The third 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 integument 

 of the lower jaw (Jig. 180, a a). 



The sixth nerve (jig. 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, D, d) : the stem of this ossicle is 

 articulated with a triangular plate of bone (r), 

 representing, according to Meckel, the incus. 

 This bone is connected with a small bent os- 

 seous style (6), 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 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. 

 The meatus is remarkably long ; the tube is 

 strengthened in this Monotreme 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, ). The tym- 

 panic fossa is almost entirely encircled with a 

 slender hoop of bone (Jig. 169, c) consisting of 

 the anchylosed tympanic bone and malleus. 



The portion which represents the tympanic 

 bone (), and which can be separated from the 

 malleus in theyoungsubject, is a slender osseous 

 filament bent into three-fourths of a circle, 

 and placed upon the inner margin of the tym- 

 panic fossa, its concavity looking outwards: 

 this concavity is impressed with a fine groove 

 for the insertion of the membrana tympani : 

 the posterior part of the hoop passes across 

 the commencement of the Eustachian canal, 

 and terminates in a free point upon the pos- 

 terior wall of the tympanic fossa : the anterior 

 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- 

 tained within the cavity of the tympanum ; the 

 principal portion forms the external and part 

 of the posterior boundary of the bony meatus 

 auditorius, and is then continued forwards in 

 the form of a slender pointed process ; the 

 bone slightly expands as it 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- 

 flected portion a slender compressed process (r) 

 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 nearly 

 a line apart: the short and simple columelliform. 

 stapes ((/) ascends vertically from the inner- 

 most of these tubercles, with the upper surface 

 of which it is articulated ; its opposite ex- 

 tremity closes the foramen ovale in the form 

 of an expanded plate. The membrana tym- 

 pani is concave outwardly at its middle part. 



The eighth and ninth pairs of nerves 

 have the usual origins and proportions. The 

 pneumogastric nerve (Jig. 180, d) is closely 

 attached, at its origin, to the hypoglossal 

 (Jig. 180, 6), but is quite distinct from the 

 sympathetic (jig. 180, f) : it gives off the 

 superior laryngeal, and then proceeds along 

 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 



