318 THE SKELETON OF THE RABBIT. 



small bone (stylo-hyal) attached to the par-occipital 

 process. The posterior cornua are the lower ends, 

 and only persistent parts, of the first pair of bran- 

 chial arches. 



5. Apertures on the surface of the skull. 



After the several bones have been examined, the 

 positions of the more important foramina should be 

 determined, most of which have already been seen. 

 Except when otherwise stated, the several apertures 

 are paired. 



a. Foramina for the passage of cranial nerves. 



i. The optic foramen is a large round median hole 

 in the orbito-sphenoids : it transmits the optic 

 nerves, and in the dry skull puts the two 

 orbits in communication with the cranial 

 cavity and with each other, 

 ii. The foramen lacerum anterius, or sphenoidal 

 fissure, is a vertical slit below and behind the 

 optic foramen, and between the basi-sphenoid 

 and ali-sphenoid. Through it the third, fourth, 

 and sixth nerves, and the ophthalmic and 

 maxillary divisions of the fifth, enter the 

 orbit from the cranial cavity. In most mam- 

 mals the maxillary division of the fifth nerve 

 leaves the skull by a separate aperture, the 

 foramen rotundum, which in the rabbit is 

 confluent with the sphenoidal fissure. 

 iii. The internal orbital foramen perforates the 

 inner wall of the orbit, about a quarter of an 

 inch in front of the optic foramen, and at the 

 anterior end of a groove which curves back 

 over the optic foramen towards the sphenoidal 

 fissure. Through it the nasal branch of the 

 ophthalmic division of the fifth nerve passes 

 from the orbit into the cranial cavity, which 

 it enters just behind the lower edge of the 

 cribriform plate. 



