SKELETON 109 



The ethmoid complex consists of a mesethmoid which ossifies in the septum 

 between the nasal organs, and an ectethmoid in the outer wall of each nasal 

 apsule. Mes- and ectethmoids are distinct for a time, the olfactory nerve 

 .)assing between them. Later bony strands passing between the nerve fibres 

 unite them, producing a perforated cribiform plate, characteristic of the 

 mammals. The part of the mesethmoid projecting above the cribiform plates 

 is the cristi galli, below them is the perpendicular plate. Two other centres in 

 the lateral wall of each capsule give rise to coiled bones (inferior and sphenoidal 

 turbinal) on which the olfactory membrane is spread, while two other turbinals 

 (superior and middle) arise from the ectethmoid. A few mammals have in 

 addition, a prenasal bone, developed in the septum in front of the mesethmoid. 



The temporal complex consists of squamosal, otic bones and tympanic. On 

 the ventral side of the squamosal is the glenoid fossa for the articulation of the 

 lower jaw; in front the bone gives off a zygomatic process for articulation with a 

 similar process of the zygomatic (malar) bone, the two forming the arcade bound- 

 ing the temporal fossa. The tympanic (apparently the angulare of the lower 

 vertebrates) curves below the auditory meatus, (fig. 80) joining the squamosal 

 on either side. In many forms it expands to form a large capsule, the auditory 

 btiUa. The otic bones (it is said that there are six centres of ossification in 

 the otic capsule) unite early to form a single petrosal bone, which, in turn 

 (cetacean excepted) fuses with squamosal to form the temporal bone. Later, the 

 posterior part of the otic region expands to form the mastoid process, while 

 the upper part of the hyoid, fused to the capsule, forms a styloid process. 



On account of the great size of the brain some parts of the skull are changed 

 in position. Thus the petrosal, instead of forming part of the side wall, is 

 carried to the floor of the brain cavity and the squamosal forms part of the 

 lateral wall. The roof of the brain cavity is largely formed by parietals and 

 frontals. (In some whales, denticetes, the supraoccipital and interparietal 

 extend to the frontal, preventing the parietals from meeting.) The frontals 

 may be distinct or they may fuse. In many ungulates they bear horns or 

 antlers. In cattle, antelopes, sheep and goats (cavicornia) a strong bony process 

 or horn core is developed on each frontal, and this is covered by a cornified 

 epidermis and persists through life. The antlers of the deer differ from horns. 

 Each year there is an outgrowth of bony material, covered by a richly vascular 

 skin, from each frontal bone. This grows with remarkable rapidity, and when 

 its full extent is reached, the skin ('velvet') is lost, leaving the core alone. 

 After about a year resorption takes place at the base so that the antler is soon 

 lost, to be replaced by a similar but larger one in a few weeks. 



The nasals lie above and behind the nares. The margin of the upper jaw 

 is formed by premaxillaries, followed by the maxillaries which ossify from 

 several centres, difficult to homologize with distinct bones in the lower verte- 

 brates. The inferior turbinals fuse to the medial surfaces of the maxillaries. 

 Premaxillaries and maxillaries may fuse or they may remain distinct. They 

 have broad palatine processes on the oral surface, these meeting in the middle 

 line and forming the anterior part of the hard palate, with frequently one or 

 two incisive foramina for the passage of the nasopalatine nerve between them. 

 The choana; are usually behind the palatine bones which form the rest of the 



