620 CHORDATA. 



them. In the floor of the cranium the sphenoid bone lies in front 

 of the basioccipital portion of the occipital. In many mammals 

 this consists of an anterior and a posterior portion throughout life ; 

 in man this condition occurs at least in the embryo. Each of 

 these parts in development consists of three elements, the posterior 

 of the basisphenoid as the body, and the paired al [sphenoids (great 

 wings); the anterior is similarly composed of the presphenoid and 

 the paired orbitosphenoids (lesser wings) (fig. 562, Spb, Ps, Ah, 

 Ors). In front of the sphenoid lies the ethmoid, Eth, likewise 

 formed from three parts, the mesethmoid, which forms a partition 

 between the two nasal cavities, and the paired ectethmoids, which 

 form the lateral walls of the nasal cavities. These last have com- 





-fia. 

 os. 



Fio. 647. Skull of embryo Tatusia. (After Parker, from Wiedersheim.) Cartilage 

 dotted, membrane and membrane bones lined, o, incus (quadrate); de, dentary; 

 /r, frontal; h, (above) membrane over anterior fontanelle, (below) hyoid bones; 



iw, premaxillary: ju, jugal (malar) ; kb, remnants of gill arch; to, lachrymal; 

 m/f, Meckel's cartilage; mx, maxillary; n, malleus (articulare); rw, nasal: <>, oc- 

 cipital cartilage; os, supraoccipital; pa, parietal ; pe, petrosal ; sg, squamosal ; .sf, 

 stapes; tj/, tympanic. 



plicated folds on their inner surface, the superior and middle 

 turbinated bones, which support the olfactory membrane, thus 

 greatly increasing its surface. With these is associated the os tur- 

 binale, a distinct bone, the inferior turbinated bone of human 

 anatomy. 



The temporal bone, which is intercalated between the roof and 

 floor of the skull, can only be understood by its embryonic rela- 

 tions and its connexion with the first and second visceral arches 

 (fig. 647). Its centre is formed by the petrosal (pe), developed 

 in the walls of the otic capsule, to which, as elsewhere in the 

 vertebrates, are attached: (1) the cartilaginous jaw arches, the 

 quadrate (a), and the mandibular (n and mlc} ; (2) the cartilaginous 



