70 VERTEBRATE SKELETON 



basisphenoid bone extends forwards in the cranial floor from the basi- 

 occipital as far as the tip of the notochord. The presphenoid, also 

 in the floor, reaches from the basisphenoid to the ethmoid region 

 and hence is largely trabecular in origin. The basisphenoid bears 

 on its anterior dorsal surface the posterior wall (dorsum sellae) o:^ the 

 sella turcica. Anterior to either anterolateral angle of this bone an 

 alisphenoid bone ossifies in the lateral wall of the cranial cavity from 

 the posterior ends of trabecula and sphenolateral cartilages and in 

 front of the exit (foramen lacerum) of the fifth and seventh nerves. 

 The presphenoid, in the same way, is associated with an orbito- 

 sphenoid bone on either side, also developed in the same cartilages, 

 the optic nerve either passing through it (optic foramen) or between 

 it and the alisphenoid in the orbital fissure. The presphenoid forms 

 the floor of the hypophysial fossa and extends forwards to the 

 ethmoid region. 



The ethmoidalia include a mesethmoid ossifying in the ethmoid 

 plate and its dorsal extensions (not mentioned above) while in or 

 near the lateral wall of each nasal capsule is an ectethmoid, parts of 

 which, in higher Vertebrates, become coiled to form conchae (turbi- 

 nates) which support the olfactory membrane. 



A series of otic bones (fig. 73) ossify in each otic capsule which 

 becomes intruded in the angle between basi- and exoccipital behind, 

 and basi- and ahsphenoid in front. Most Vertebrates have three of 

 these otica, a prootic in the anterior part, an epiotic in the roof and 

 an opisthotic in the posterior cupula, pro- and opisthotic usually 

 meeting on the inferior side of the capsule. Fishes may have two 

 more otica, a sphenotic in front of the others and a postero-lateral 

 pterotic, lying over the semicircular canal. Whether there be any 

 traces of these last in higher Vertebrates is uncertain. In the higher 

 classes the otic bones fuse in the adult to a single petrosal or periotic 

 bone. There is also a tendency for the petrosal to unite with the 

 squamosal (a membrane bone described later) to form the temporal 

 bone of mammals. The only other cranial cartilage bones are the 

 sclerotics (p. 62) in the outer part of the sclera of the eye. 



The cartilage visceral skeleton (especially the first two arches) 

 undergoes considerable modification in the higher Vertebrates. In 

 all classes above the Elasmobranchs and Chondrostei the pterygo- 

 quadrate no longer is the skeleton of the upper jaw, this being formed 

 by membrane bones to be described below. With this change in 



