54S 



CHORDATA 



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

 walls of the i^iasal ca\iiies. These last ha\-e complicated folds on their 

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

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

 associated the o.^ tiirhinah', a distinct bone, the inferior turbinal bone of 

 human anatomy. 



The temporal bone, which is intercalated between the roof and floor 

 of the skull, can only be understooil by its em])r\-onic relations and its 

 connexion \\ith the iirst and second \isceral arches (lig. 59S). Its centre 



Fig. 59S. — Skull of embrvo Talusia (after Parker, from ^^'ie^lersllei^l). Cartil- 

 age dotted, membrane and membrane bones lined, a, incus (quadrate); ile, dcntari,'; 

 _/>, frontal; /;, (above) membrane over anterior fontanelle, (below) hyoid bones; iin, 

 prcmaxillap,-; /», jugal (malar); kb, remnants of gill arch; !<i, lacrimal; 'iik, >[eckers 

 cartilage; w;.v, maxillary; ii, malleus (artieulareV, lui. nasal; o. occipital cartilage; os, 

 supraoccipital; l>a, parietal; pi\ petrosal; 5(/, squamosal; si, stapes; ty, t\"mpianic. 



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

 to wliich, as elsewhere in the vertebrates, are attached: (1) the cartilag- 

 inous jaw arches, the quadrate (a), and mandiliular (;/ and nik); {2) the 

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

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

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

 sal (sq), at tlie liase of the cjuadrate, which increases as tlie latter loses 

 in size, and l)elow the siiuamosal the lympanie {ly). With ossification 

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

 with the sciuamosal, 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- 



