SKELETON. 



677 



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

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



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



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

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

 basioccipital beneath, the exoccipitals on the sides, the supraoccipital 

 above. The exoccipitals form most of the occipital condyles, but the 

 basioccipital contributes a small part. In many Mammals the ex- 

 occipitals alone form the condyles. From each exoccipital a parocci- 

 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. 



Along the roof of the skull from 

 behind forwards lie the supraoccipital, 

 the parietals, the frontals, and the 

 nasals. Between the supraoccipital 

 and the parietals there is a small in- 

 lerparietal. 



On the very front of the skull are 

 the premaxillic, bearing the incisor 

 teeth. Behind each premaxilla is a 

 maxilla, bearing the premolars and 

 molars ; behind this, along the zygo- 

 matic or temporal arch projecting 

 beneath the orbit, is \.\\.zjngal or malar, 

 which unites posteriorly with the sqita- 

 mosal. This zygomatic arch bridges 

 over the deep temporal fossa behind 

 the orbit, and serves for the insertion 

 of muscles, and its " squamoso-max- 

 illary" structure occurs outside of 

 Mammalia in the Anomodont reptiles 

 only. The fact that in Rodents the 

 malar does not form part of the face is 

 of considerable systematic importance. 

 The sqiiamosals form a great part of 

 the posterior side walls of the skull, 



and articulate with the parielals, frontals, orbitosphenoids, and ali- 

 sphenoids. At the posterior end of the zygomatic arch is the longi- 

 tudinally 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 

 basioccipital ; 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 



Pmx. 



skull. 



S.O., Top of supraoccipital; //., 

 interparietal ; T., tympanic ; Pa., 

 parietal; Si/., squamosal ; Fr., 

 frontal; /., jugal ; *\'a., nasal; 

 Pinx., premaxilla. 



