COMPOSITION OF THE SKULL. 



379 



ing gill-slits or spiracles, but are closed in the higher Verte- 

 brates. As a rule, the skull is symmetrical, exceptions being 

 found in the flounders and the bones about the nose of cer- 



Fig. 381. -Skull of the Lion. 2, occipital condyle : 7, Parietil bone and sagittal 

 crest ; 8, piroccluital ; 27', squamosal bone ; 27, zygomatic arch ; 26, malar bone ; 

 11, frontal bone; 12, poet-orbital process; 15, nasal bone; 21, maxillary bone; 22, 

 premaxillary biine ; .32, mandible ; 3, occipital crest ; c, canine teeth ; p'^, second pre- 

 molar ; ml, molar tooth.— After Owen. 



tain whales and porpoises. The base of the skull is perfo- 

 rated for the exit of the nerves proceeding from the base of 

 the brain, and the hinder bone {occiput) is perforated {fora- 

 men magnum) for 

 the passage of the 

 spinal cord from the 

 medulla oblongata. 

 It is probable that 

 there is a general 

 parallelism between 

 the head of Insects 

 and Vertebrates. 



While the head of Fig. 382.— SkuIl and vl^ceial skeleton of a Selachian 



(diagram), occ, occipital region; la, wall of the laby- 



Wmged insects, for rinth ; eth, ethmoidal region ; n, nasal pit ; a, flrst, 6, c, 



. . J, second labial cartilage ; o, sunerior, n, inferior portion 



example, consists Ot of the mandibular arch /. ; //., hyoid arch; IIL-VIII. 



r; ■ 1, i! (1-6), branchial arches.- After Gegenbaur. 



a certain number or 



segments, homologous with those of the rest of the body, 

 and with mouth-parts homologous with the limbs ; so the 

 skull is also segmented, and an expansion and continuation 

 of the vertebral column. Gegenbaur even maintains that 

 the various arches of the head are homologous with the limbs. 



