SKELETON. 67 



dalia, there being two sets of sphenoidalia. The occipitalia arise in the 

 occipital vertebrae and in the basilar plate. Of these there are four 

 (figs. 66, 67) : A supraoccipital above, an exoccipital on either side, 

 and a basioccipital below, the latter extending forward into the basilar 

 plate. These four form a ring around a central opening, the foramen 

 magnum, through which the spinal cord connects with the brain. 



Just in front of the basioccipital the basilar plate ossifies to form the 

 basisphenoid, which extends forward to the sella turcica, and there is 

 succeeded by the presphenoid, arising from the trabeculae, and ex- 

 tending forward to the ethmoid plate. On either side a bone, the 

 alisphenoid, ossifies in the cartilage of the same name, and comes into 

 close relation with the basisphenoid. Father hi front a second element, 

 the orbitosphenoid, arises in the alisphenoid cartilage and comes into 

 relation to the presphenoid. The alisphenoid bone is just in front of 

 the otic capsule, but there is always a large gap (sphenoidal fissure, 

 foramen lacerum anterior) between it and the orbitosphenoid, through 

 which the third, fourth, and sixth and the ophthalmic branch of the 

 fifth nerve pass, the rest of the fifth nerve passing through the alisphe- 

 noid bone. The optic nerve usually perforates the orbitosphenoid, 

 but may pass through a notch in its margin. 



The ethmoid plate may ossify into a median mesethmoid bone 

 bounded on either side by an ect ethmoid and in some there may be 

 added other bones included among the 'turbinal bones.' The ol- 

 factory nerves pass on either side of the mesethmoid, the ectethmoids 

 (below) in the mammals developing as perforated plates (cribiform 

 plate). 



A series of otic or petrosal bones is developed in each otic cap- 

 sule. The most constant of these are a prootic in front, an opisthotic 

 behind, the two usually meeting below (fig. 66), and between them, 

 above, an epiotic, concerning which more evidence is needed. In 

 the teleosts and some other forms the lateral wall of the otic capsule 

 may develop in addition a sphenotic in front and a pterotic behind, 

 the latter overlying the horizontal semicircular canal of the ear. In the 

 higher groups the various otic bones fuse in the adult to a single petro- 

 sal bone, which is wedged in between the lateral parts of the basi- 

 occipital and basisphenoid. 



In the stegocephals, reptiles and birds the sclera often gives rise to 

 a ring of sclerotic bones (fig. 67), which, however, never unite with 

 the other bones of the skull. The nasal capsules often develop a 



