22 LABORATORY DIRECTIONS 



mandibular fossa, a notch for the articulation of the mandible. 



The tympanic region consists of the tympanic bulla ventrally 

 (already mentioned), and the ring of bone surrounding the 

 external auditory meatus. 



The mastoid region includes the heavy, thickened portion 

 posterior to the external auditory meatus. It lies between the 

 squamous region and the bulla. 



The petrous region is seen best on the inner surface of the 

 temporal bone. From this side it is the central mass of very 

 dense bone (hence the name, meaning stony) between the 

 squamous region and the tympanic bulla. In it are embedded 

 the structures of the inner ear. 



Turn now to the ventral surface of the skull and again locate 

 the premaxillaries and the maxillaries. Then look for the 

 following: 



11. Palatines. — The two palatines form that portion of 

 the roof of the mouth behind the maxillaries. Each palatine 

 has a vertical plate, forming part of the wall between orbit and 

 nasal cavity and extending posteriorly to meet processes of the 

 sphenoid, 



12. Vomer. — This is a single bone seen by looking into the 

 anterior nares. It is a slender, elongated bone in the floor of 

 the nasal cavity, and has a groove on its dorsal surface. At its 

 posterior end it articulates with the body of the presphenoid. 

 Only this posterior end is visible on the ventral surface. 



13. Presphenoid. — This bone is made up of two portions, 

 which are distinct in young kittens and in lower vertebrates. 

 These parts are the base (presphenoid of lower forms) and the 

 two wings (orbitosphenoids). The base lies in the roof of the 

 mouth just posterior to the vomers and is partly covered by 

 the vomer, the palatine, and the sphenoid bones, so that only a 

 narrow central strip is visible. The two wings come from nearly 

 the whole dorsal-lateral angles of the basal portion. They 

 appear on the surface as part of the median walls of the orbits, 

 ventral to the frontal bones and posterior to the palatines. 

 They carry the optic foramina for passage of the optic nerves. 



14. Sphenoid. — In man this is joined with the presphenoid 

 to form a single bone; in the cat, however, it is separate. It is 

 formed by the union of five bones which are found in lower 



