THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 119 



trated. It forms part of the anterior wall of the orbit, ventral to the lacrimal 

 bone, by its orbital process; it extends to the frontal bone above by its frontal 

 process; its palatine process meets its fellow in the median ventral line continuing 

 the hard palate; its alveolar process bears teeth; and its zygomatic process con- 

 stitutes the beginning of the zygomatic arch. The malar or jugal bone is distinct 

 in the cat but in the adult rabbit is fused to the zygomatic process of the maxilla. 

 The temporal bone is a compound bone characteristic of mammals. It consists 

 of a squamous portion which by its zygomatic process completes the zygomatic 

 arch and which also contributes to the cranial wall, ventral to the parietal; 

 of the tympanic bulla composed of a tympanic bone of uncertain homology; and 

 of the periotic or petromastoid bone only slightly visible on the surface and con- 

 sisting of the fused otic bones of lower vertebrates. The squamous part of the 

 temporal bone is homologous with the squamosal bone of the lower vertebrates 

 and is the most posterior bone of the maxillary arch. All of the bones of the 

 maxillary arch are membrane bones. 



The median portion of the upper jaw forms part of the ventral surface of 

 the skull. We have already noted that the anterior portion of the hard palate 

 is composed of the palatine processes of the premaxillae and maxillae. Posterior 

 to the latter are the palatines, membrane bones which include the posterior nares. 

 Dorsal to the palatines the roof of the nasal passages is completed by the vomer 

 of which only the extreme posterior tip is visible in the roof of the choanae. 

 The vomer forms the floor of the nasal cavities and is best seen in a sagittal section 

 of the skull. The posterior parts of the palatines extend up into the orbit, 

 there being in the rabbit a very deep cleft between them in the mid ventral line. 



Observe that the quadrate is wanting. The lower jaw consequently articu- 

 lates with the squamosal (temporal) by means of a depression, the mandibular 

 fossa, on the under surface of the zygomatic process of the temporal bone. This 

 feature distinguishes mammalian skulls from those of all other vertebrates. 

 Owing to the absence of the quadrate all of the bones of the upper jaw are mem- 

 brane bones. 



4. The occipital region. — This region surrounds the foramen magnum and 

 consists of a single occipital bone extending from the superior nuchal line to a 

 point between the anterior ends of the tympanic bullae. It is really composed 

 of the four occipital bones present in reptiles; these are distinct in embryonic 

 and young mammals, as shown in Figure 40 A , page 1 2 7, but are fused in the adults. 

 The occipital bone bears the two occipital condyles, whose number serves to dis- 

 tinguish mammalian skulls from those of all other vertebrates except Amphibia, 

 where there are also two condyles. The dorsal part of the occipital may appear 

 to extend anteriorly between the posterior ends of the two parietals; this is 

 due to the fact that interparietal bone is commonly fused to the occipital. This 

 is the case in man. The occipital bone bears the superior nuchal line and the 

 jugular process resting on the bulla. 



