74 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



bones of the cartilaginous brain case may be arranged in four groups, 

 beginning behind and called respectively occipitalia, sphenoidalia 

 and ethmoidalia, there being two sets of sphenoidalia. The occipi- 

 talia arise in the occipital vertebrae and in the basilar plate. Of these 

 there are four (figs. 74, 75): 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 con- 

 nects with the brain. 



• F]^' 74-— Ventral view of schematic skull, chondrocranium dotted, cartilage bones 

 with lines and dots, basioc, basioccipital; basisph, basisphenoid; als, alisphenoid; exoc- 

 exoccipital; ors, orbitosphenoid; presph, presphenoid; premax, premaxilla; qti, quadrate 

 quju, quadratojugal; squamos, squamosal; zygom, zygomatic; other names in full. 



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

 the basisphenoid, which extends forward to the sella turcica (p. 68), 

 and is there succeeded by the presphenoid, arising from the trabe- 

 culae, and extending 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. Farther in 

 front a second element, the orbitosphenoid, arises in the ahsphenoid 

 cartilage and comes into relation to the presphenoid. The alisphe- 

 noid 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 



