COMPOSITION OF THE SKULL. 



399 



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- 



i'i?. 386.— Skull of the Lion. 2, occipital condjie ; 7, Parietal bone and sagittal 

 crest ; 8, pf.rocci.iital ; 2?', squamosal bone ; a?, zygi>matic arch ; 26, malar bone ; 

 11, ttontal bone ; 12, post-orbital process; 15, nasal bone; 21, maxillary bone; 22, 

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

 molar ; nil, 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 [ocvipid) is perforated {fora- 

 men magnum) for 

 the passage of the 

 spinal cord from the 

 medulla oblongata. 

 It is probable that 

 there is a general 

 parellelism between 

 the head of Insects 

 and Vertebrates, 



tp thnt while the Fig. 387.— Skull and visceral skeleton of a ^iacAian 

 I.e., xnat wniie uie (^,^|^^j^^ ^^^ occipital region; la, wall of the laby- 

 head of insects, for rinth : elh, ethmoidal region ; n, nasal pit ; a, ftrst, b. e, 

 second labial cartilage ; o, suoerior, n, inlerior portion 

 example, consists OI of the mandibular arch/. ; //., hyoid arch; III.-VIII. 

 f- . 1 „ (1-6), branchial arches.— After Qegenbaur. 



a certain number or 



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

 and with mouth-parts homologous with the limbs ; tha,t the 

 skull is also segmented, and an expansion and continuation 

 of the vertebral column. Gegenbaur even maintains tiiat 

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



