SKULL — MAMMALS 1 89 



on the line between basi- and presphenoid. In Monotremes and 

 Marsupials the internal carotid artery perforates the basisphenoid; 

 elsewhere it enters the cranium between petrosal and aHsphenoid, 

 frequently enclosed in a canal on the former bone. Basisphenoid 

 and basioccipital sometimes fuse, but usually fusion occurs between 

 the basi- and presphenoid, although they are often distinct. The 

 presphenoid (especially prominent in rodents) bears the sulcus for 

 the optic chiasma, and sometimes is excluded from contact with the 

 brain by the meeting of the orbitosphenoids above it. It occasion- 

 ally fuses with the mesethmoid in front. 



The two pairs of sphenoidal wings always fuse with basi- and 

 presphenoid. In Primates the alisphenoid is the larger of the two, 

 but elsewhere the size relations are reversed, rendering inapph- 

 cable the terms ala magna and ala parva of human anatomy. The 

 great width of the brain brings both wings largely to the floor of the 

 cranial cavity, but they also form a part of the side wall and con- 

 tribute to that of the orbit. Each aHsphenoid ossifies from two 

 centres, one for the greater part of the bone, the other in the carti- 

 lage inner lamella of the pterygoid process, the outer forming the 

 larger part of the bone. These two lamellae (with a pterygoid groove 

 between them) are strong descending processes from the lower side 

 of the bone, the inner lamella being closely associated with the 

 pterygoid bone of the adult. The relations of alisphenoid to the 

 cranial nerves vary. In many there are foramina ovale and rotun- 

 dum for the third and second rami of the trigeminal nerve respect- 

 ively, but ovale and lacerum are continuous in many species, while 

 in most groups rotundum and orbital fissure are not separate. 



The orbitosphenoid has a single centre of ossification (fig. 199); 

 it is separated from the aHsphenoid by the orbital (sphenoidal) 

 fissure through which, in many mammals, optic, together with the 

 third, fourth, sixth and the ophthalmic ramus of the fifth, nerves 

 pass. But in most higher groups the optic has its own foramen 

 which passes obHquely through the bone, the foramina of the two 

 sides being confluent in some rodents. 



The ethmoidaHa arise in the septum and lateral walls of the nasal 

 capsule, the olfactory nerves entering the capsule through two 

 foramina in the cartilage separating nasal and cranial cavities. This 

 region (lamina cribrosa) is strongly incHned or nearly vertical in 

 Ornithorhynchus and many lower groups, but with increase of the 



