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- 



Fig. 386. -Skull of the Lion. 2, occipital condyle ; 7, Parietal bone and sagittal 

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

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

 premaxillary bone ; 32, mandible ; 3, occipital crest ; c, canine teeth ; />*, second pre- 

 niolar ; 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 

 parellelism between 

 the head of Insects 

 a n d Vertebrates, 



i.e., that while the Fig. 387. Skull and visceral skeleton of a Selachian 



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



head OI insects, lor rintli ; etfi, ethmoidal region ; n, nasal pit ; a, first, b, c, 



, i j> second labial cartilage ; o, suoerior, n, inferior portion 



example, Consists Ot of the mandibular arch /. ; //., hyoid arch; HI.-VIII. 



a certain number of ( 



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

 and with mouth-parts homologous with the limbs ; that 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. 



