SKELETON. IOI 



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, pre- 

 venting 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 vertebrates. The 

 inferior turbinals fuse to the inner 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 choanae are usually 

 behind the palatine bones which form the rest of the hard palate, but in some eden- 

 tates and whales the pterygoids form part of the partition between the narial pas- 

 sages and the mouth cavity. 



The ingrowth of the hard palate has forced the vomer from the roof of the mouth 

 to a position just ventral to the anterior part of the cartilage of the nasal septum. 

 In the monotremes there is a 'dumb-bell 

 bone' in front of the vomer (p. 69). A 

 lacrimal bone always occurs at the inner 

 side of the orbit and the zygomatic forms 

 the external wall of that cavity. 



The lower jaw articulates directly with 

 the squamosal without the intervention of 

 a quadrate (see ear bones, p. 74). Its pc 



halves may unite in front by ligament or ., r , / ,. 



FIG. 105. Hyoid of rhinoceros (Ate- 



by complete anchylosis. It is usually hdus). ac, anterior cornu; b, body; c, 

 described as consisting of a pair of den- ceratohyal;, epihyal:c, posterior cornu 

 taries, but there are several centres of ossi- 

 fication and a splenial and possibly a coronoid may be recognized. The angulare 

 is apparently the tympanic, while the articulare of lower vertebrates is the malleus. 

 A remarkable feature in development is an enormous cartilage at the posterior 

 angle of the jaw, the dorsal side of which forms the condyle for articulation with 

 the glenoid fossa. 



The hyoid apparatus varies. As described above, the hyoid is connected above 

 with the otic region, below with the first branchial. The part connected with the 



