276 SKELETON OF THE BABBIT 



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 

 determine ; it is probably developed from the 

 uppermost 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 partly of cartilage -bones, which 

 replace these arches, but principally of membrane - 

 bones developed around and in front of them. 

 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- 

 sphenoid. On the outer side of the pterygoid, 

 between it and the 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 boundary of the pterygoid fossa. 

 They form' the side- walls of the posterior 



