304 THE SKELETON OF THE RABBIT. 



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. 



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. Bones in relation with the maxillary arch. These 

 are very closely connected with the cranium and 

 with the olfactory capsules. 



i. The pterygoids are a pair of narrow vertical 

 plates of bone, attached to the base of the 

 skull at the junction of the basi-sphenoid with 

 the ali-sphenoids. The posterior border of 

 each pterygoid is free, and ends below in the 

 curved hamular process, while the anterior 

 border unites with the palatine and ali-sphe- 

 noid. On the outer side of the pterygoid, 

 between it and the external pterygoid process 

 of the ali-sphenoid, is the triangular pterygoid 

 fossa. 



ii. The palatines are a pair of nearly vertical plates 

 of bone, attached above to the ventral border 

 of the pre-sphenoid, and behind to the ptery- 

 goids and ali-sphenoids, thus completing the 

 anterior boundaries of the pterygoid fossee. 

 They form the side-walls of the posterior 



