2o ANATOMY OF VERTEBRATES. 



like condition, would thus be affected by any sonorous vibrations 

 that might be propagated to the tympanic cavity ; and the share 

 which the cochlea would take in their application to the acoustic 

 nerves may explain the large proportional size of that part of the 

 labyrinth and of the foramen rotundum. 



In Sirenia the acoustic capsule is small, but dense in structure ; 

 it coalesces with the tympanic and mastoid, and the compound 

 ear-bone is partly lodged in a large hemispheric cavity of the 

 squamosal, and partly projects into the wide vacuity between 

 that bone, the basispheiioid, and basioccipital. The otosteals 

 are relatively large, especially the stapes, fig. 170, E (Manatus), 



which forms a massive, elon- 

 gate, conical, subcompressed 

 ossicle, truncate atop and ob- 

 liquely perforated above its 

 oval convex base : the incus is 

 a much smaller bone with one 

 crus thick, the other short and 

 styliform : the malleus has a 



Stapes of aquatic mammals. * 



A. ornithorbynchus. B. Porpoise, c. walrus. large irregularly globose head 



D. Seal. E. Manatee. i i 11 11 



and a handle terminated by an 



abrupt point. The massiveuess of the malleus of the Porpoise, 

 ib. B, and Walrus, ib. c, has already been referred to : in the Seal, 

 ib. D, the bone has lost less of the character of the mammalian 

 stapes. In the Ornithorhynchus the avian type, ib. A, is retained, 

 and the prolongation of the column has not developed the pro- 

 cesses marking out the incus, as in the marsupial, fig. 171. 



In Monotremata the acoustic nerve is expended upon a laby- 

 rinth remarkable for the small relative size of the semicircular 

 canals and the free projection of their bony wall into the cranial 

 cavity. The cochlea is wide, but not high ; it is bent in two 

 turns, divided as usual into a vestibular and tympanic scala. The 

 foramen ovale is nearly circular and opens into the wide but 

 shallow tympanic cavity. It is closed by the base of the columel- 

 liform stapes, fig. 170, A; the incus being represented by a pro- 

 longation and expansion of the handle or neck of the columella, 

 as in Birds. In this class such incudial expansion is joined to an 

 obtuse angled triangular plate of cartilage, the longest side of 

 which is attached to the membrana tympani. In the Orni- 

 thorhynchus the homologue of this cartilage is ossified, forming a 

 bent plate which performs the office of the manubrium of the 

 malleus and also contributes to the frame of the membrana tym- 

 pani. This membrane is concave externally, and forms the inner 



