SKULL 427 



basisphenoid, a rhomboid-shaped bone with the narrow side 

 anterior ; its upper surface is thickened in the middle, and in the 

 thickening is a depression, the sella turcica, in which lies the 

 pituitary body (p. 445). Arising from the basisphenoid on each 

 side is an alisphenoid, which here forms the side walls of the 

 cranium. In front of the basisphenoid is a narrow presphenoid, and 

 on each side of this an orbitosphenoid, which continues the side 

 wall of the cranium. Presphenoid and orbitosphenoids are not 

 recognisable as separate bones. The front wall of the skull is made 

 by the cribriform plate, a wall of bone pierced with many holes 



11 iiiv'V'v,/- ; h. 



IV ^ I 



VI 



Fig. 338. — The left side of a dog's skull, from which the zygomatic arch 



has been cut away. 



as., Alisphenoid; c, canine tooth; e.a.m., external auditory meatus; jr., frontal; g.f., glenoid fossa; 

 %., incisor teeth ; i.o.f., infraorbital foramen ; 7., jugal ; lac, lacrimal ; m., molar teeth ; mx., maxilla; 

 rla., nasal ; o.c, occipital condyle (on exoccipital bone) ; os., orbitosphenoid ; pa., parietal ; pal., 

 palatine ; pm., premolar teeth ; pmx., premaxilla ; pt., pterygoid ; s.o., supraoccipital (with which is 

 fused an interparietal) ; sq., squamosal ; sut., line of dots marking the suture between the jugal and 

 squamosal on the removed zygomatic arch ; t., tympanic ; 1 1. -VI., foramina for cranial nerves. 



for the olfactory nerve ; it is part of the mesethmoid, which 

 also has a median vertical extension forwards, the nasal septum. 

 The cartilage bones leave the roof of the brain uncovered. This 

 gap is filled in with membrane bones : a pair of large parietals 

 above the alisphenoids, a pair of large frontals above the orbito- 

 sphenoids, a pair of small lacrimals below and in front of the 

 frontals, and a small unpaired interparietal above the supra- 

 occipital. Part of the wall of the skull between the exoccipital 

 and the alisphenoid is made by the auditory capsule. It is 

 sometimes helpful to learn the main bones of the cranium 

 as three rings, with no distinction of cartilage and mem- 

 brane bones. The occipital ring consists of supra-, basi-, and 

 exoccipitals ; the parietal ring of basi- and alisphenoids and 



