284 SKELETON OE THE EABBIT 



small fossa on the inner surface of the 

 tympanic bone. 

 /8. The incus is a minute, somewhat pyriform 

 bone : its head bears in front a saddle- 

 shaped articular surface for the body of 

 the malleus, and is produced behind into 

 a short process, which articulates with a 

 fossa on the outer surface of the periotic 

 bone, just above the fenestra ovalis. 



The narrow end or stalk of the incus 

 projects downwards : its tip is bent in- 

 wards, and has attached to it a very small 

 bony disc, the os orbiculare. 

 y. The stapes is a very small stirrup-shaped 

 bone, articulating by its arch with the os 

 orbiculare, and attached by its basal plate to 

 the membrane closing the fenestra ovalis. 

 The morphological value of the auditory 

 ossicles has been much debated, and is not 

 yet determined with certainty. The incus 

 and malleus are probably developed from the 

 cartilage of the mandibular arch ; the incus 

 being derived from the uppermost part of the 

 arch, and corresponding to the quadrate bone of 

 other vertebrates ; while the malleus is formed 

 from the part of the arch below the incus. 



The stapes is still more difficult to deter^ 

 mine ; it is probably developed from the upper- 

 most part of the cartilage of the hyoidean arch. 

 3. The jaws. The cartilaginous maxillary and mandibular 

 arches of the embryo, corresponding to the upper and 

 lower jaws of the dog-fish, disappear almost com- 

 pletely during development ; the jaws of the adult 

 rabbit consisting of membrane-bones developed 

 around and in front of these arches. 

 a. Sones in relation with the maxillary arch. These 

 are very closely connected with the cranium and 

 with the olfactory capsules. 



