anterior margin of the orbit). Frequently in the cat, a small 

 exposure of ethmoid forms the orbit wall between the frontal 

 and lacrimal. The alisphenoid is excluded from contact with 

 the frontal by the large orbitosphenoid in the rabbit. Ante- 

 rior to the orbit the frontal sutures with the maxilla (in front) 

 and nasal (above). 



The nasals are paired splints of bone lying above the nasal 

 passages. They contact the frontals posteriorly and the max- 

 illae and premaxillae laterally. They form a part of the 

 margin of the external narial opening. 



The lateral and ventral margins of the external narial 

 opening are formed by the premaxillae (singular pre- 

 maxilla) bearing the teeth called incisors (three on each side 

 in the cat and dog; two pairs, one in front of the other, in 

 the rabbit). The premaxillae form the anterior part of the 

 palate. The slim palatal processes meet along the midline 

 between the large anterior palatine fenestrae. These proc- 

 esses in the rabbit do not contact the maxillae but meet a 

 downward projecting process of the vomers. 



Behind the prema.xilla is the maxilla. Each ma.xilla bears 

 teeth. In the cat or dog there is a large canine anteriorly. In 

 front of the canine there is a gap in the tooth row called a 

 diastema into which the canine of the lower jaw fits. Behind 

 the canine are "shearing teeth," three (premolars) in the cat 

 and four (premolars) in the dog. The most posterior of these 

 teeth in both animals is called a carnassial tooth, since its 

 cutting edge matches that of a tooth in the lower jaw in such 

 a way that they function like scissors. The other shearing 

 teeth pass between each other, cutting by means of a slid- 

 ing action. 



The dog has two molars behind the carnassial. The cat 

 has one. The rabbit lacks incisors and has six peg-like teeth 

 in each maxilla; its formula is 1 Vi, C %, P %, M %. The 

 formula indicates the number of incisors (I) in the upper 



jaw over the number in the lower jaw, the number of 

 canines (C), premolars (P), and molars (M) in a similar 

 fashion. 



The paired maxillae meet at the midline of the secondar>' 

 palate; they suture with the premaxillae anterioriy and the 

 palatines posteriorly. On the lateral aspect the maxilla 

 sutures with the jugal below the orbit margin (by means of 

 a zygomatic process), touches the lacrimal along the anterior 

 margin of the orbit and sutures with the frontal above this, 

 contacts the nasal dorsoanteriorly and the premaxilla ante- 

 riorly. Within the orbit the maxilla sutures medially with 

 the palatine and the lacrimal; laterally and anteriorly it con- 

 tacts the jugal. At the lower anterior margin of the orbit, 

 the maxilla is pierced by the large infraorbital canal. 



The posterior part of the secondary palate is formed by 

 the palatines lying just behind the maxillae. These bones 

 nearly wrap around either side of the nasal passages and 

 form the secondary palate below those passages. Dorsally 

 the palatines are separated by the vomer. The palatine of 

 either side also extends up into the orbit to suture with the 

 frontal, the lacrimal, and the maxilla. In the orbit of the 

 cat the palatine is pierced by the large sphenopalatine fora- 

 men and more laterally by the posterior palatine canal. In 

 the dog the posterior palatine foramen lies immediately 

 below the sphenopalatine foramen. These foramina are 

 hidden behind the swollen maxilla in the rabbit. 



The lacrimal is a thin plate of bone in the anteroventral 

 part of the orbit. Its anterior margin forms the margin of 

 the orbit and is pierced by the large downwardly and in- 

 wardly directed lacrimal canal. The lacrimal is so loosely 

 attached that sometimes it is lost in prepared skulls of the 

 cat; frequently it is missing in the rabbit. 



Behind the palatine and forming the lateral walls of the 

 nasal passages are the pterygoids. The pterygoid of either 



frontal 



parietal 



optic foramen' 

 orbital fissure 

 olor canal 



squamosal 



postparietol 



orbital fissuri 

 eustachian tube 



petrosal 



mastoid foramen 

 postglenoid foramen 

 mastoid process 



occipital condyle 



exoccipital process 



tympanic aperture 

 postglenoid process 



pterygoid 



stylomastoid foramen' 

 bulla' 



anterior palatine 

 foromino 



nternol choana 



arrow through 

 olor canal 



foramen ovale 

 postglenoid process 



postglenoid foramen 

 mastoid process 

 exoccipital process 

 hypoglossal foramen 



posterior lacerate fossa 



fora 



men magnum 



B 



Figure 3-3. Posterior port of skull of dog as seen laterally, A, and ventrolly, B. 



MAMMALIAN HEAD SKELETON • 39 



