The Skeletal System. 



55 



mals), paired parietals, squamosals, frontals, and nasals as 

 roofing bones, and paired premaxillary, maxillary, palatine, 

 pterygoid, and mandibular bones forming the solid supports of 

 the mouth. Several lateral elements also take part, including the 

 lacrimal, at the anterior border of the orbit, the zygomatic, 

 forming the central portion of the corresponding arch, for pro- 

 tection of the orbit and muscular support, and finally the bladder- 

 like tympanic bone, which forms the enclosure of the middle ear 

 and protects the delicate bones of the auditory chain. 



(k 



cb 



v • 



Fig. 29. Lateral view of skull of rabbit foetus, 45 mm. cb, co, en, cranial, 

 orbital, and nasal portions of primary chondrocranium; fr, frontal; i, incus; 

 ip. interparietal; m, malleus; mn, mandible; mx, maxilla: na, nasal;, pa, 

 parietal; pi. palatine; pmx. premaxilla; sq, squamosal; st, styloid process; t, 

 tympanic; zy, zygomatic; Born plate model, after Yoit. 



An important though inconspicuous portion of the head skeleton 

 is formed by the hyoid apparatus supporting the tongue, and 

 certain cartilages of the larynx, with which the hyoid is intimately 

 associated. The relation of this complex to the skull is indicated 

 in a mammal by the suspension of the hyoid apparatus from its base. 

 The malleus, incus and Stapes of the auditory chain form with 

 the elements just described that portion of the visceral skeleton 

 as modified in the mammalian skull, with the exception, as described 

 below, of certain replacing or derm elements also considered to 

 belong to this division. 



