SKELETON OF THE RABBIT. 551 



ciated arches, membrane-bones developing in the investing 

 skin and not preformed in cartilage. In the following 

 account, the names of the membrane-bones are printed in 

 italics. 



We have already noticed the chief characteristics of the 

 mammalian skull, such as the usual persistence of sutures, 

 the two condyles, the bony palate, the fusion of the periotic 

 bones, the articulation of the mandible with the squamosal, 

 the fusion of the parts of each ramus of the mandible into 

 a single bone in the adult, and the three ossicles of the 

 internal ear. 



In studying the skull it is convenient to consider the bones in groups. 



Along the roof of the skull from behind forwards lie the supra-occipi- 

 tal, the parietals, the frontals, and the nasals. Between the supra- 

 occipital and the farietals there is a small interparietal. 



On the posterior surface of the skull, the foramen magnum, through 

 which the spinal cord issues from the cranial cavity, is bounded by the 

 basi-occipital beneath, the ex-occipitals on the sides, the supra-occipital 

 above. The ex-occipitals form most of the occipital condyles, but the 

 basi-occipital contributes a small part. In many mammals the ex- 

 occipitals alone form the condyles. From each ex-occipital a par-occi- 

 pital process descends and is applied to the tympanic bulla — a dilatation 

 at the base of the tympanic bone which protects the external auditory 

 tube. 



On the very front of the skull are the premaxillce beariug the incisor 

 teeth. Behind each premaxilla is a maxilla, bearing the premolars and 

 molars, behind this, along the zygomatic or temporal arch projecting 

 beneath the orbit is \!a& jugal ox malar which unites posteriorly with the 

 squamosal. This zygomatic arch bridges over the deep temporal fossa 

 behind the orbit, and serves for the insertion of muscles, and its 

 " squamoso-maxillary " structure occurs outside of Mammalia in the 

 Anomodont reptiles only. The squamosals form a great part of the 

 posterior side-walls of the skull, and articulate with the parietals, 

 frontals, orbitosphenoids, and alisphenoids. At the posterior end of 

 the zygomatic arch is the longitudinally elongated glenoid cavity in 

 which the mandible moves backwards and forwards. 



In connection with the floor of the skull and the roof of the mouth, 

 there lie from behind forwards the following components : — the median 

 basi-occipital ; the median basisphenoid which lodges the pituitary body 

 in a dorsal depression called the sella turcica ; the paired alisphenoids 

 fused to the sides of the basisphenoid ; the median presphenoid which 

 forms the lower margin of the optic foramen between the two orbits ; 

 the paired orbitosphenoids, fused to the presphenoid, sutured to the 

 alisphenoids and squamosals, and surrounding the optic foramen ; the 

 vertical pterygoids attached at the junction of basisphenoid and alisphen- 

 oids ; the nearly \er\.ica\ palatines, united above to the presphenoid and 

 behind to the pterygoids and alisphenoids, separating the posterior nasal 

 passages from the orbits, and uniting in front to form the posterior part 



