36 GENERAL ANATOMICAL CHARACTERS 
occipital segment consists of the basioccipital, exoccipital, and 
supraoccipital bones; the middle segment of the basisphenoid, ali- 
sphenoid, and parietal bones; and the anterior segment of the 
presphenoid, orbitosphenoid, and frontal bones. The axis is 
continued forwards into the mesethmoid, or septum of the nose, 
around which the bones of the face are arranged in a manner 
so extremely modified for their special purposes that anatomists 
who have attempted to trace their serial homologies with the more 
simple portions of the axial skeleton have arrived at very diverse 
interpretations. The characteristic form and structure of the face 
of mammals is mainly dependent upon the size and shape of (1) the 
orbits, a pair of cup-shaped cavities for containing the eyeball and 
its muscles, which may be directed forwards or laterally, placed 
near together or wide apart, and may be completely or only partially 
encircled by bone; (2) the nasal fosse, or cavities on each side of 
the median nasal septum, forming the passage for the air to pass 
between the external and the internal nares, and containing in their 
upper part the organ of smell; (3) the zygomatic arch, a bridge of 
bone for the purpose of muscular attachment, which extends from 
the side of the face to the skull, overarching the temporal fossa ; 
(4) the roof of the mouth, with its alveolar margin for the implanta- 
tion of the upper teeth. The face is completed by the mandible, or 
lower jaw, consisting of two lateral rami, articulated by a hinge 
joint with the squamosal (a cranial bone interposed between the 
posterior and penultimate segment of the brain-case, where also the 
bony capsule of the organ of hearing is placed), each being composed 
of a single solid piece of bone, and the two united together in the 
middle line in front, at the symphysis,—which union may be per- 
manently ligamentous or become completely ossified. Into the 
upper border of the mandibular rami the lower teeth are implanted. 
In addition to the bones already mentioned as entering into the 
formation of the cranium, there are many others, the most import- 
ant of which may be briefly noticed. The anterior extremity of the 
skull is formed by the premaxille (Figs. 6, 7, PJZc), which carry the 
incisors; behind them are the maxille, in which all the remaining 
upper teeth are implanted. Both the premaxille and maxille meet 
in a median suture on the palate, where they form a floor to the nasal 
passage ; this floor being continued backwards by the plate-like pala- 
tines, at the hinder extremity of which the posterior nares are usually 
situated. In a few instances, however, as in certain Edentates and 
Cetaceans, the small pair of bones forming the posterior continuation 
of the lateral borders of the palatines, and known as the pterygoids 
(Fig. 6, Pt), likewise meet in the middle line below the nasal passage, 
and thus cause the aperture of the posterior nares to be situated 
near the occiput. On the upper, or frontal aspect of the cranium the 
paired nasals roof over the nasal passage and fill the interval left 
