tf GENERAL ANATOMICAL CHARACTERS 



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occipital segment consists of the basioccipital, exoccipital, and 

 supraoccipital bones ; the middle segment of the basisphenoid, ali- 

 sjmenoid, 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 

 Avho 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 fossa?, 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 scpiamosal (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 premaxillse (Figs. 6, 7, PMx), which carry the 

 incisors ; behind them are the maxilla?, in which all the remaining 

 upper teeth are implanted. Both the premaxillse and maxilla? 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 



