THE SKELETON 21 



often designated as bones (e.g., squamous bone) probably 

 because these portions are separate in young mammals and are 

 actually distinct bones in lower vertebrates. The tympanic 

 portion contains the external and middle parts of the auditory 

 organ and presents two striking external structures. One of 

 these, the mastoid process, is a large protuberance of spongy 

 bone which occupies most of the caudolateral side of the skull 

 craniad to the occipital bone and dorsad to the tympanic 

 bulla. The other, the tympanic bulla, is a large, bulbus, 

 sacuiate bone with a large external foramen, the external 

 auditory meatus. The petrous portion of the temporal is a 

 very solid, stone-like bone which lies mediad to the bulla and 

 contains the meatus, semicircular canals, cochlea, vestibule, 

 foramen for the eighth cranial nerve and other structures of 

 the internal ear. Its medial margin is visible through the 

 foramen magnum. The squamous portion of the temporal 

 bone forms most of the side of the cranium and gives off the 

 zygomatic process, which articulates with the malar bone to 

 form the zygomatic arch. 



The two frontal bones articulate with each other, with the 

 parietals and squamous portion of temporals caudad, the 

 nasals, premaxillaries, maxillaries, and lachrymals craniad and 

 with other bones. The frontal bones form part of the vault 

 of the cranium and of the nares and nearly the entire superior 

 portion of the orbits, where each gives off a prominent supra- 

 orbital process. 



The two nasal bones form the major part of the roof of the 

 nares, and form visible articulations with the frontals and 

 frontal process of the premaxillaries. 



The premaxillaries bear the two large and two small 

 incisors, articulate dorsally with the nasals and frontals, 

 caudally with the maxillaries and internally with the vomer and 

 maxiloturbinals. 



The maxillaries bear the grinding teeth, form the caudal 

 half of the incisive foramen and articulate medially with the 

 palatines and lachrymals, laterally with the malar bones (which 



