276 MAMMALIA. 



of the skull in place of the interorbital septum extend forward into 

 the ethmoid region. Thus it happens that the ethmoid (lamina j 

 cribrosa) constitutes the boundary of the anterior and lower part 

 of the skull (fig. 668). The temporal bones also take an essential 

 part in bounding the cranial cavity, since not only the petrous and a 

 part of the mastoid,* but also the large squamosal occupy the space 

 remaining between the alisphenoids and exoccipitals. The occipital 

 always articulates with the first cervical vertebra (atlas) by two 

 condyles, and its lateral portions (exoccipitals) frequently present a 

 pyramidal process on each side (Jugular or paramastoid' process). 

 The prsesphenoid and basisplienoid (fig. 668) often remain separate 



IT 



Sf 



FIG 668. Median longitudinal section of sheep's skull, from inside. Ob, Basi-occipital 

 Ol, exoccipital ; Os, supra-occipital ; Pe, petrous ; Spb, basi-sphenoid ; Ps, prae> 

 sphenoid; Als, alisphenoid ; Ors, orbito-sphenoid ; Pa, parietal ; Fr, frontal; <Sy, frontal 

 sinus ; Eth, ethmoid ; Na, nasal ; C, ethmoturbinal ; Ci, inferior turbinal ; ft, pterygoid ; 

 Pal, palatine ; Vo, vomer ; MX, maxillary ; Jmx, praemaxillary. 



for a long time. To the latter are applied the alisphenoids with the 

 parietals, which belong to this region. An interparietal is often 

 developed behind the parietal ; it is, however, usually ankylosed with 

 the supra-occipital, more rarely with the parietal. The frontal bones 

 constitute the roof of the skull in the region of the orbitosphenoids ; 

 they are less frequently fused than are the parietals. The temporal 

 bone has several constituents (1) The petrous portion, which is 

 composed of the three pieces of the periotic capsule the pro-, opistho-, 

 and epiotic ; (2) the mastoid portion, which is a part of the epiotic ; 



(3) the squamous portion or squamosal, which is a larger bony scale ; 



(4) the tympanic bone, which is attached to the squamosal, bounds 



* [The petrous and mastoid together constitute the periotic.] 



