XIII 



PHYLUM CHORDATA 453 



squamosal, are the tympanic and pcriotic bones. The tympanic 

 forms the bony part of the wall of the external auditory meatus ; 

 below it is dilated to form a process {ty. 1ml.) projecting on the 

 under surface of the skull — the hiilla tympani. The pcriotic is a 

 bone of irregular shape, its internal {pctrmm) portion (Fig. 108.S, 

 2n'ri) enclosing the parts of tlui membranous labyrinth of the 

 internal ear, and externally presenting two small openings — the 

 fenestra ova/is and fenestra rotunda — visible only when the tym- 

 panic is removed ; internally it bears a depression, the fiomilar 

 fossa, for the lodgment of the flocculus of the cerebellum. Part 

 of the periotic (mastoid portion) is seen on the exterior of the 

 skull between the tympanic and exoccipital. The periotic and 

 tympanic are not ankylosed together, and are loosely connected 

 with the surrounding bones, being held in position by the post- 

 tympanic processes of the squamosal. Between the tympanic and 

 periotic are two foramina of importance — the stylomastoid, which 

 transmits the seventh cerebral nerve, and the Eustaehian ojperture, 

 at which the Eustachian tube opens. 



Roofing over the olfactory cavities are two flat bones — the nasals 

 (nas.) — each having on its inner surface a very thin, hollow process, 

 the naso-turhinal. In front of the nasals are the premaxillte 

 {p. max) — large bones which form the anterior part of the snout, 

 bear the upper incisor teeth, and give off three processes — a nasal, 

 a palatine (pal. j^. max), and a maxillary. The maxilla: (^max.),\\\uc\i 

 form the greater part of the upper jaw, and bear the pre-molar and 

 molar teeth, are large, irregularly-shaped bones, the outer surfaces 

 of which are spongy. They give off internally horizontal processes 

 — the palatine processes 0?f^/. max) — which unite to form the 

 anterior part of the bony palate. Between the premaxilhe and 

 maxillae and the palatines on the lower surface of the skull is a 

 large triangular opening divided into two — the anterior palatine 

 foramina — by the palatine processes of the premaxilla-. On the 

 outer surface of each maxilla, above the first pre-molar tooth, is 

 a foramen — the infra-orbital (i.o. fo) — through which the second 

 division of the fifth nerve passes. A strong process which is given 

 off from the outer face of each maxilla, and turns outwards and 

 then backwards to unite with the zygomatic process of the 

 squamosal and thus complete the zygomatic arch, is a separate 

 bone in the young — the malar ox jiigal (jn.). 



The maxilte help to bound the nasal cavities externally, and with 

 each is connected on its inner aspect a pair of tliin scroll-like 

 bones — the maxillo-turhinals (Fig. 1083, mx. th.). The rest of the 

 narrow bony palate, forming the roof of the mouth and the floor of 

 the nasal cavities, is formed by ihe i^alatine plates of the palatine 

 hones (pal.). The pterygoids (/>.) are small irregular bones, each of 

 which articulates with the palatine in front and with the pterygoid 

 process of the alisphenoid behind. The lacrymals (Icr.) are small 



