side extends down and back as a slim process, the hamulus. 

 The pterygoids are separated at the midline by the "presphe- 

 noid," which upon examination is seen to be but the mid- 

 line area of fusion of the bilateral orbitosphenoids. The 

 basisphenoid is exposed between the posterior parts of the 

 pterygoid. The suture of the pterygoid with the palatine is 

 not sharp; posterolaterally it is fused with the alisphenoid 

 in the cat or rabbit but not in the dog. 



In the roof of the nasal passage, between the pterygoids 

 and palatines, the posterior bifurcated end of the fused 

 vomers is seen. In the cat or dog the vomers form a splint 

 of bone which is V-shaped in cross section. The anterior end 

 of this midline splint lies above the posterior tips of the 

 palatal processes of the premaxillae, except in the rabbit. 

 This anterior tip can be seen through the anterior narial 

 opening. The vomerine unit of the rabbit is more exposed 

 and has a deep ventral keel extending down as a process 

 behind the palatal processes of the premaxillae. The more 

 dorsal body of the fused vomers extends well beyond and 

 above the posterior ends of these processes. 



The aliiphenoid forms a part of the cranial wall extending 

 upward and outward from the area of fusion of the ptery- 

 goid. The ahsphenoid is pierced by the foramen rotundum 

 (in front) and the foramen ovale (behind). The large orbital 

 fissure opens between the alisphenoid and the orbitosphe- 

 noid. In the dog the first two foramina are joined by a hori- 

 zontal channel through the alisphenoid, the alar canal (Fig- 

 ure 3-3). In the rabbit a foramen ovale opens under the 

 anterior extension of the bulla, and the fenestra rotundum 

 is confluent (together with) the orbital fissure. The alar 

 canal of the dog, which transmits the internal maxillary 

 artery, is represented in the rabbit by a foramen through the 

 orbital wing of the pterygoid. 



Above the orbital fissure the alisphenoid sutures with the 

 orbitosphenoid, above and lateral to this with the frontal 

 and parietal; posterolaterally it contacts the squamosal, the 

 suture passing just inside the glenoid articulation of that 

 bone. In the rabbit the squamosal and orbitosphenoid 

 margin the alisphenoid in the orbit. The posterior limit of 

 the alisphenoid is hidden by the tympanic bulla; medially 

 it is fused with the basisphenoid and pterygoid. 



In the orbit, anterior to the alisphenoid and the orbital 

 fissure, is the orbitosphenoid. This bone forms a part of the 

 forewall of the cranial capsule. It is margined above by the 

 frontal, in front and below by the palatine, and behind by 

 the alisphenoid. This bone is pierced by the large optic 

 foramen. The orbitosphenoid is largest in the rabbit, where 

 it sends a process out laterally above the alisphenoid. This 

 process reaches the squamosal. There is a small process in 

 the cat that does not contact the squamosal. In the dog the 

 orbitosphenoid is smallest. The bilateral orbitosphenoids 

 fuse at the midline and their palatal exposure is identified 

 as the "presphenoid." Since a presphenoid as a separate 

 identifiable bone is not know to occur in any mammal, or 

 lower form, and since the orbitosphenoid of lower forms may 



arise from lateral and medial centers of ossification as in the 

 mammal, it is suggested that the term presphenoid be 

 dropped. 



The basisphenoid is separated by distinct sutures from the 

 orbitosphenoid anteriorly and the basioccipital posteriorly; 

 it is fused laterally with the alisphenoid and sometimes the 

 pterygoid. A small hypophyseal fenestra penetrates this 

 bone at the midline in many cats and in the rabbit. 



The tympanic bullae are formed in part from the tym- 

 panic bone, which encircles, except for a part of the dorsal 

 margin (the notch of Rivinus), the tympanic membrane. 

 The outer margin of the tympanic is drawn out and upward 

 in the rabbit to form an external auditory meatus. This 

 bone also extends medially to form part of the bulla. In the 

 cat the tympanic forms a septum separating the bullar 

 cavity into an ossicular cavity and a bullar recess. In the 

 dog this septum is only partly developed. The medial part 

 of the bulla of the cat or dog ossifies in a cartilaginous pre- 

 cursor called the entotympanic. 



The surface of the bulla medial to the mastoid process is 

 grooved by the connective tissue of the hyoid attachment to 

 the skull. In the rabbit and young cat, this attachment is 

 ossified. This ossification, the stylohyal, is not fused to the 

 bulla. In man the stylohyal is fused to the petrosal as the 

 styloid process. It ossifies from two centers, the tympanohyal, 

 next to the petrosal, and the stylohyal. 



The tympanic bulla is penetrated near its medial margin 

 by a carotid canal in the rabbit. In the foetal cat such a canal 

 lies between the bulla and the basisphenoid but is vestigial 

 in older stages. In the dog the internal carotid artery enters 

 the carotid canal in the posterior lacerate fossa. Both the 

 carotid canal and its internal portal (two openings) can be 

 seen when looking at the anterior aspect of the bulla along the 

 palatal surface. From this view, the opening of the eustachian 

 tube and the nearly confluent petrotympanic fissure open 

 lateral to the internal carotid portal. In the cat, only the eu- 

 stachian tube and petrotympanic fissure are seen, whereas in 

 the rabbit the foramen ovale opens above the eustachian tube 

 and petrotympanic fissure. 



The cat skull cut sagittally (Figure 3-2 C) reveals many 

 details of structure. The secondary palate extends below the 

 large nasal passage, which is nearly filled with the thin, 

 contorted and rolled sheets of bone making up the turbinals. 

 These are identified according to their area of attachment 

 as the maxillary, nasal, and ethmoid turbinals. The per- 

 pendicular plate of the ethmoid separates the right and left 

 nasal passages and joins the cribriform plate separating the 

 nasal passages from the cranial cavity. The turbinals, eth- 

 moid and cribriform plates are secondary ossifications 

 appearing late in development. A section of the frontal and 

 orbitosphenoid bones reveals sinuses connecting with the 

 nasal passages. Such sinuses also are found in the maxilla, 

 and in some mammals more of the bones of the skull are 

 invaded by such air cavities. 



The large cerebral fossa of the brain case is separated 



40 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



