COMPOSITION OF THE SKULL. 



431 



Elements. 



I. Parachordals 

 and trabeculae, 

 aided in some 

 cases by the end 

 of the notochord. 



Origin. 



Results. 



Their precise 

 relations, e.g. , to 1 

 the notochord are I 

 unknown. 



III. Arches. 

 [a) Mandibular. 



{b) Hyoid arch. 



IV. Investing 

 membrane bones, 



[a) From the 

 roof of the skull. 



[b] On the floor 

 of the skull, i.e., 

 from the roof of 

 the mouth. 



{c) About the 

 sides of the skull. 



((/) About the 

 upper jaw. 



{e) About the 

 lower jaw. 



These arches, 

 like those which 

 follow them, are 

 supports of the 

 pharynx, lying 

 between primitive 

 or persistent gill 

 slits. Perhaps 



they may be com- 

 pared to ribs. 



Occipital region, with four bones 

 — basi-occipital, two ex-occipitals, 

 and a supra-occipital (in part). The. 

 basi-occipital is distinct only in 

 Reptiles, Birds, and Mammals. 



Sphenoidal and ethmoidal region, 

 with basi-sphenoid and pre-sphenoid, 

 paired alisphenoids and orbitosphen- 

 oids, the inter-orbital septum, the 

 lateral or ectoethmoids, the inter- 

 nasal septum. 



Originally of 

 the nature of ex- 

 ternal bony 

 plates, tooth 

 structures, and 

 the like. 



[a) Unite with ethmoidal region. 



\b) May give origin to five bones — 

 pro-, sphen-, pter-, epi-, and opisth- 

 otics, or to the single periotic of Alani- 

 mals. 



{a) Upper part = palato-pterygo- 

 quadrate cartilage of Elasmobranchs, 

 palatine, pterygoid, and quadrate 

 bones in the higher Vertebrates, but 

 in Mammals the quadrate is believed 

 by many to become the incus of the 

 inner ear. 



Lower part^Meckel's cartilage — 

 the basis of the lower jaw in all 

 animals; the part next the quadrate 

 becomes the articular bone, which in 

 Mammals is believed by many to 

 become the malleus of the inner ear. 



{p) Upperpartorhyo-mandibular — 

 the " suspensorium " cartilage of 

 Elasmobranchs, the hyo-mandibular 

 and symplectic of Teleosteans, the 

 columella auris of Amphibians, Rep- 

 tiles, and Birds, the stapes of the 

 Mammal's ear. 



Lower part = the hyoid proper 

 (cartilage or bone). 



{a) Parietals, frontals, nasals, iS:c. 



[b) Vomer, parasphenoid, &c, 



[c] Lachrymal, squamosal, orbitals, 

 &c. 



id) Premaxilla, maxilla, jugal, and 

 quadrato-jugal (in part). 



[e) Dentary, splenial, angular, 

 supra-angular, coronoid. 



