548 



CHORDATA 



nasal cavities, and the paired ectethmoids, which form part of the lateral 

 walls of the nasal cavities. These last have complicated folds on their 

 inner surface, the superior and middle turbinal bones, which support the 

 olfactory membrane, thus greatly increasing its surface. With these is 

 associated the os turbinale, a distinct bone, the inferior turHnal 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 relations and its 

 connexion with the first and second visceral arches (fig. 598). Its centre 



la: 



FIG. 598. Skull of embryo Tatusia (after Parker, from Wiedersheim). Cartil- 

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

 fr, frontal; h, (above) membrane over anterior fontanelle, (below) hyoid bones; -im, 

 premaxillary; ju, jugal (malar); kb, remnants of gill arch; la, lacrimal; mk, Meckel's 

 cartilage; mx, maxillary; n, malleus (articulare); na, nasal; o, occipital cartilage; os, 

 supraoccipital; pa, parietal; pe, petrosal; sq, squamosal; st, stapes; ty, tympanic. 



is formed by the petrosal (pe}, developed in the walls of the otic capsule, 

 to which, as elsewhere in the vertebrates, are attached: (i) the cartilag- 

 inous jaw arches, the quadrate (a), and mandibular (n and mk)', (2) the 

 cartilaginous hyoid arch, the stapes (in part equalling the hyomandibular, 

 st), and the hyoid proper (//) (compare with the visceral skeleton of the 

 selachian, fig. 546). To these are added the membrane bones, the squamo- 

 sal (sq), at the base of the quadrate, which increases as the latter loses 

 in size, and below the squamosal the tympanic (ty). With ossification 

 of the cartilaginous parts several centres form the petrosal, which fuses 

 with the squamosal, and frequently with the tympanic, which in some 

 forms enlarges to a conspicuous auditory bulla. Petrosal and squamosal 

 on the one side, tympanic on the other, enclose a space, the tympanic 

 cavity, into which the upper parts of both visceral arches extend, ossify- 



