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. 
Inthe mammalia generally the teeth are differentiated into incisors, 
canines, premolars and molars, and in placental mammals the 
full dental formula is indicated as i. 3,c. 4, pm. 4,m. 3. Inthe 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. 2, c. ®, pm. 3, 
m. 3. 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 
DENTITION. 
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, 
