COMPOaiTION 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 tlie flounders and the bones about the nose of cer- 



Pig. 381. - Sknll of the Lion, 2, occipital coniiyle ; 7, Parietil bone and sagittal 

 ereut ; 8, paroccipitaJ ; 27', sq^namosal bone ; 27, zygomatic arch ; 26, malar bone ; 

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

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

 molar ; m/, 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 ^'g' 382.— Skull and Ti^cersil skeleton of a Selachian 



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



Winged insects, for rlnth : eth, ethmoidal region ; n, nasal pit ; o, first, b, c, 



, - I j» second labial cartilage ; o, superior, n, inferior portion 



example, consists Ol of the mandibular arch/. ; 11., hyoid arch; HL-VIII. 



, . -u j> (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. 



