The Skeletal System. 59 



As accessory structures the teeth of the rabbit present two 

 mammalian features; they are heterodont, or differentiated 

 according to particular regions ; and the adult series, excepting those 

 designated as molars, are permanent teeth, replacing deciduous, 

 or milk teeth of the young animal. The condition is thus de- 

 scribed as diphyodont in comparison with that in lower verte- 

 brates, where there is usually a multiple tooth change, new teeth 



being developed as required (polyphyodont type). 



Moreover, in the rabbit, as in all mammalia, the 

 number is restricted, so that, considering the differentiation of the 

 teeth, it is possible to express their relations by a dental formula. 

 In the mammalia generally the teeth are differentiated into incisors, 

 canines, premolars and molars, and in placental mammals the 

 full dental formula is indicated as i. ;.;, c. \ , pm. | , m. ::. In the rabbit 

 as in other rodents, however, the dentition is greatly modified 

 by the elaboration of two pairs of incisors for gnawing and the 

 corresponding obliteration of intermediate teeth, the place of the 

 latter being occupied by an extensive gap. or diastema, in which no 

 teeth occur. The dental formula of the rabbit is i. >', c. ;;. pm. f, 

 m. :;. It will also be observed in this animal that the absence of the 

 intermediate teeth allows the lips to be approximated behind the 

 incisors, and since in this region the lips are also provided with 

 hairs on their internal surfaces, the oral cavity is separated almost 

 completely from a small space enclosing the incisor teeth. This 

 adaptation, however, is not so perfectly developed in the rabbit 

 as in certain others of the rodent order. 



The human skull differs most markedly from that of the rabbit 

 or other mammal in the enormous development of the cranial 

 region, and in the anteroposterior compression of the face, with 

 THE HUMAN which is associated, shortening of the jaw region, 

 SKULL. reduction of the nasal cavities and rotation of the 



orbits to a forward position. A most instructive 

 feature is the re-adjustment of the axis, coincident with the assump- 

 tion of the erect position.' In both quadrupedal and bipedal 

 positions the face naturally retains its forward direction. In most 

 mammals, as in vertebrates generally, the axial line of the cranium, 

 known as the basicranial axis, and that of the face, the basifacial 

 axis, tend to be continuous or at least parallel; while in man, 



