THE SKULL. 

 Composition of the Skull. 



37 S 



Elements. 



Origin. 



Results. 



I. Parachordals 

 and trabeculae, aided 

 in some cases by the 

 end of the notochord. 



Their precise re- 

 lations, e.g.., to the 

 notochord, are un- 

 known. 



Occipital region, with four bones — basi- 

 occipital, two ex-occipitals, and a supra- 

 occipital (in part). The basl-occipital is 

 distinct only in Reptiles, Birds, and 

 Mammals, 



Sphenoidal and ethmoidal region, with 

 basi-sphenoid and pre-sphenoid^ paired 

 alisphenoids and orbitosphenoids, the 

 inter-orbital septum, the lateral or ecto- 

 ethmoids, the inter-nasal septum. 



II. Sense-capsules, 

 (a) Nasal. 



ip) Auditory. 



From cartilage 

 surrounding the ec- 

 todermic pits which 

 form the foundation 

 of nose and ear. 



(a) unite with ethmoidal region. 



(^) May give origin to five bones— pro-, 

 sphen-, pter-, epi-, and opisth-otics, or to 

 the single periotic of Mammals. 



III. Arches, 

 (a) Mandibular. 



{b Hyoid arch. 



These arches, like 

 those which follow 

 them, are supports 

 of the pharynx, 

 lyingbetween prim- 

 itive or persistent 

 gill-slits. Perhaps 

 they may be com- 

 pared to ribs. 



id) Upper part = palato-pterygo-quad- 

 rate 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 arti- 

 cular bone, which in Mammals is believed 

 by many to become .the malleus of the 

 inner ear. 



{J>) Upper part or hyo-mandibular=the 

 *' suspensorium " cartilage of Elasmo- 

 branchs, the hyo-mandibular and sym- 

 plectic of Teleosteans, the columella auris 

 of Amphibians and Reptiles, the stapes of 

 the Mammal's ear. 



Lower part=the hyoid proper cartilage 

 or bone. 



IV. Investing mem- 

 brane bones. 

 ia) From the roof 

 of the skull. 



ip) On the floor of 



the skull, z'.f., from 



theroof of the mouth. 



(c) About the sides 



of the skuU. 



id) About the up- 

 per jaw. 



ie) About the lower 

 jaw. 



Originally of the 

 nature of external 

 bony plates, tooth 

 structures, and the 

 like. 



id) Parietals, frontals, nasals, etc. 

 iU) Vomer, parasphenoid, etc. 



ic) Lachrymal, squamosal, orbitals, etc. 



i^d) Premaxilla, maxilla, jugal, and 

 quadrato-jugal (in part). 



ifi) Dentary, splenial, angular, supra- 

 angular, coronoid. 



