296 



MARSUPIALIA. 



.Fig. 11 9. 



grey matter enveloping the central medullaiy 

 tract in each lateral moiety of the chord is 

 shewn in the three situations marked 1, 2, and 

 3 (fg. 119); the superior expansion and com- 

 plexity of the grey matter in the anterior 

 columns of the pelvic enlargement accords 

 with the predominance of the locomotive over 

 the sensitive function* in the strong saltatory 

 hind legs of the Kangaroo. 



Organs of Sense. The olfactory nerves and 

 the osseous cavities and laminae destined for 

 the protection and support of the pituitary 

 membrane offer a remarkable proportional 

 development in all the Marsupials, and more 

 especially in the Insectivorous and Carnivorous 

 tribes. The outer root of the olfactory nerve 

 appears as a continuation of the whole nati- 

 form protruberance ; a corresponding accumu- 

 lation of nervous matter," protuberantia py- 

 riformis,"f at the base of the inner root of 

 the olfactory nerve has already been noticed ; 

 both these broad tracts consist externally of 

 grey matter : the middle or medullary root of 

 the nerve is the smallest. It is very con- 

 spicuous in the Wombat, where it emerges 

 from a longitudinal fissure at the anterior and 

 inner part of the " protuberantia natiformis." 

 The olfactory nerves or rather lobes are hollow, 

 and contain, as in most Mammalia, the ante- 



Fig.120. 



Stapes of the Kan- 

 garoo. 



' Mayo, Outlines of Physiology, p. 235. 



'tiwv/, VUkllllCO VI J^ I 



Phil. Trans. 1837, p. 



rior continuation or extremity of the lateral 

 ventricles (o, fig. 118). The filaments pass 

 from the skull through several foramina of a 

 cribriform plate. Certain species of Kangaroo, 

 of the subgenus Osphranter, Gould, remarkable 

 for their acuteness of smell, have the turbinated 

 bones so large that the lateral expansion of the 

 nasal cavity forms a marked feature in the skull. 

 The chief deviation from the Mammalian 

 type in the organ of hearing presents itself in 

 the form of the stapes, the body of which is 

 a simple elongated style either entirely, as 

 the Perameles, or for half its length, as in 

 the Kangaroo (jig. 120), where it divides into 

 two short crura, before ter- 

 minating in the base ; it thus 

 more or less approximates to 

 the form of the columella in 

 the bird. 



The largest proportional 

 external ears are those of the 

 Perameles lagotis, the short- 

 est those of the Wombat. The internal struc- 

 ture of the ear is not less strikingly developed 

 in the Perameles than the outward appendages. 

 The tympanic cavity is very extensive, but is 

 formed, as in other Marsupials, by the sphe- 

 noid and petrous bones ; the tympanic bone 

 is limited to the function of supporting the 

 ear-drum, and forming the internal commence- 

 ment of the meatus auditorius externus. The 

 internal extremity of the tympanic cylinder 

 projects obliquely into the posterior and outer 

 part of the sphenoidal bulla. The membrana 

 tympani is a delicate transparent tissue, 

 slightly concave internally, and having the 

 long handle of the malleus (a, fg. 121) at- 

 tached to it. This pro- 

 cess of the malleus is bent 

 upon itself at a right an- 

 gle; the inner portion is 

 broader and thicker than 

 that which is attached to 

 the membrana tympani, 

 and it is anchylosed at its 

 internal extremity by a 

 thin and transparent plate 

 of bone (b) to the side 

 of the incus (c). This 

 little ossicle, which here 

 appears as a process of 

 the malleus, presents a 

 notched articular surface for the orbicular end 

 of the stapes. This portion of the stapes gives 

 off a short slender process for the attachment of 

 the stapedius (d), and then is continued in the 

 form of a moderately long and slender columel- 

 liform shaft to the elliptical and slightly ex- 

 panded base which closes the foramen ovale. 



Organ of Vision. The anatomy of the eye 

 offers no peculiarity illustrative of the affinities 

 of the Marsupialia or of any other speciality in 

 their economy save the nocturnal habits of the 

 majority of the order. It is in relation to these 

 habits that the lens is large and convex, the 

 iris broad, the pupil round and generally wide, 

 and the cornea correspondingly large. 



The Harderian gland and the retractor oculi 

 co-exist, as usual in Mammalia, with the. 

 nictitating eyelid. This is always largely de~ 



Fig. 121. 



Auditory ossicles, 

 Perameles. 



