510 



VERTEBRATE LIFE AND ORGANIZATION 



Alisphenoid 

 (I) 



MeSe-thmoid 



Occipital- 



y D e ntary 



— ■^ (Mandible) 



'-Meckel's carlila^^Cl) 

 'HyoidCH,™;) 



Ca.rtila0e of larynx 

 (Ei:,T, 3Zr,3ZIl) 



.Tractieal car-'bila.gzs 



Figure 25.6. Components of the human skull, hyoid and larynx. Dermal bones have 

 been left plain, chondrocranial derivatives are hatched, those parts of the embryonic 

 visceral skeleton that disappear are stippled, parts of the visceral arches that persist 

 are shown in black. Roman numerals refer to visceral arches and their derivatives. 

 (Modified after Neal and Rand.) 



sequence until reptiles. The number of sacral vertebrae has increased 

 as the tetrapods have evolved more effective terrestrial locomotion. 

 Typically amphibians have one, reptiles two and mammals three. The 

 greater number in man is probably correlated with the additional prob- 

 lems oi support inherent in a bipedal gait. 



The mammalian skull has many of the features found in the frog's 

 skull. The expanded portion housing the brain is the cranium; the jaws 

 and the bones surrounding the eyes and supporting the nose constitute 

 the facial skeleton. The eyes are lodged in orbits, the nasal cavities 

 open on the surface through external nares, an external auditory 

 meatus leads into the middle ear cavity, the spinal cord emerges 

 through the foramen magnum, and there are many smaller foramina 

 for blood vessels and nerves. A temporal fossa, in which jaw muscles 

 are lodged, lies posterior to the orbit. It is bounded laterally by a 

 handle-like bar of bone, the zygomatic arch. A bony hard palate 

 separates the mouth and nasal cavities and the internal nares lie at 

 the posterior end of this. 



The skull is a hodgepodge of cartilage replacement and dermal 

 bones that can be understood only when considered from an evolution- 



