SKULL. LOWER JAW. 485 



to the parasphenoid. There is only one temporal arcade formed 

 of the jugal and zygomatic process of the squamosal and corre- 

 sponding either to the lower temporal arcade 6f reptiles or to the 

 upper and lower temporal arcades combined into one and not 

 perforated by a lateral temporal fossa. 



The maxillary and palatine bones are firmly united with the 

 skull, and there is a hard palate formed by palatal plates of the 

 premaxilla, maxilla and palatine, and occasionally, as in croco- 

 diles, by the pterygoid (some cetaceans and edentates). The 

 pterygoid, so conspicuous in the lower forms, is a small scale- 

 like bone connected with the palatine but without relation 

 with the suspensorial region. 



The quadrate has been absorbed into the squamosal so that 

 the lower jaw articulates with the squamosal direct. The arti- 

 cular surface for the condyle of the lower jaw is called the glenoid 

 cavity. The squamosal enters largely into the side wall of the 

 skull. The lower jaw consists of the fused articulare and den- 

 tary and is formed of one piece on each side. The periotic con- 

 sists of the petrous portion (petrosal) in the skull (Fig. 260, Pe) 

 and the mastoid portion which appears on the surface between 

 the exoccipital and the squamosal (Fig. 259, Pe) ; it ossifies from 

 three centres which constitute the epiotic, prootic and opisthotic 

 elements ; these unite with each other before joining any neigh- 

 bouring bones. The periotic usually unites with the squamosa 

 and the tympanic to form the temporal bone. The tympanic 

 bone (Ty) is a membrane bone which frequently forms the floor 

 and front wall of the tympanic cavity and may be prolonged 

 outwards in a tubular or spout-like manner, bounding the bony 

 external auditory meatus. It is often considerably swollen 

 to form the tympanic bulla. The complex of bones, called 

 the temporal, fills in a gap in the skull wall between the 

 exoccipital and the alisphenoid, leaving only a small unfilled 

 portion in front and behind. The former is called the 

 foramen lacerum medium basis cranii and transmits the in- 

 ternal carotid artery ; the latter is larger and constitutes the 

 foramen lacerum poster ius basis cranii which transmits the 

 internal jugular veins, and the 9th, 10th, and llth cranial 

 nerves. 



The occipital bone always articulates with the atlas vertebra 

 by two condyles, and its lateral portions frequently possess a 



