THE SKELETAL SYSTEM 5 
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. 

Fic. 29. Lateral view of skull of rabbit foetus, 45 mm. cb, co, cn, cranial, 
orbital, and nasal portions of primary chondrocranium; fr, frontal; i, incus; 
ip, interparietal; m, malleus; mn, mandible; mx, maxilla; na, nasal; pa, 
parietal; pl, palatine; pmx, premaxilla; sq, squamosal; st, styloid process; t, 
tympanic; zy, zygomatic; Born plate model, after Voit. 
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. 
