PROTECT/ON, SUPPORT AND MOVEMENT 



511 



ary point of view. As the brain grew larger during the course of evolu- 

 tion, the cartilage replacement bones of the chondrocranium could no 

 longer completely encase it. They form a ring of bone around the fora- 

 men magnum (the occipital bone), encase the inner ear (part of the 

 temporal bone), and form the floor of the cranium. The sides and roof 

 of the cranium are completed by dermal bones such as the frontal and 

 parietals, and by a portion of the mandibular arch known as the alis- 

 phenoid (Fig. 25.6). The last is a cartilage replacement bone. 



Although the mandibular arch is associated with the jaws in most 

 vertebrates, at least to the extent of forming the jaw joint, the jaws of 

 mammals are formed entirely of certain of the dermal bones that encased 

 the mandibular arch in primitive vertebrates. The mammalian jaw joint 

 lies between two of these— the dentary and squamosal (part of the 

 temporal). The posterior end of the mandibular arch, which forms 

 the jaw joint in more primitive vertebrates, has become the incus and 

 malleus— two of the three small auditory ossicles that transmit vibra- 

 tions across the middle ear cavity. Our ancestral jaw joint is now part 

 of our hearing mechanism, and earlier it was part of a gill arch and 

 concerned with respiration! The third auditory ossicle, the stapes, 

 evolved from the dorsal part of the hyoid arch. It is of interest to observe 

 that the auditory ossicles have the same relationship to each other as 

 their homologues in fish. The ventral part of the hyoid arch, together 

 with the remains of the third visceral arch, form the hyoid bone (a 

 sling for the support of the tongue), and the styloid process of the skull 

 to which the hyoid is connected by a ligament. With the loss of gills in 

 tetrapods, the remaining visceral arches have become greatly reduced, 

 but parts of them form the cartilages of the larynx. 





. <8Z'-4; -^i :.1ff<?if.//'!..%: 



-Supcra.cl«.if;hrum 



■Sc«.puIocor«x.cid 



-Humerus 

 -Radius 



Clcithrum 



ScatpulocoratcoM* 







Phalztnges 



Figure 25.7. Lateral views of the appendicular skeleton of a crossopterygian, A, 

 and labyrinthodont, B, to show the changes that occurred in the transition from water 

 to land. Dermal bones have been left plain, cartilage replacement bones are stippled. 

 (A, Modified after Gregory; B, after Romer.) 



