THE MORPHOLOGY OF THE SKULL 119 



Metamorphosis of the Branchial or Visceral Bars 



These rods of cartilage are named, f cm before backward, the mandibular, hyoid, and 

 thyreoid bars. They may with care be easily dissected in the foetus between the third and fourth 

 months. Their metamorphosis is as follows:^ 



The two extremities of the mandibular bar (cartilage Meckelii) ossify; the distal end ulti- 

 mately forms a portion of the mandible near to the symphysis (see p. 98); the proximal end 

 ossifies as the malleus and incus. The intermediate portion disappears; the only vestige is a 

 band of fibrous tissue, the spheno-mandibular ligament, extending from the spine of the sphenoid 

 to the spine of the mandible. 



In the connective tissue surrounding the bar there appear, however, ossifications, one of 

 which invests the bar to form the dentary plate; while a second, situated more proximally, forms 

 the tympanic bone. 



The hyoid bar fuses distally with the thyreoid bar, and forms part of the hyoid bone. Its 

 proximal end becomes the stapes, the tympano-hyal portion of the styloid process (fused with 

 the petro-mastoid), and the stylo-hyal or free portion of the process. The succeeding portion 

 (epi-hyal segment) is represented in the adult by the stylo-hyoid ligament, and the lowest seg- 

 ment, or cerato-hyal, by the small cornu of the hyoid. 



The thjrreoid bar forms the great cornu of the hyoid bone (thyreo-hyal). The body of the 

 hyoid (basi-hyal) is regarded as representing the fused ventral ends of the hyoidean and thy- 

 reoidean arches. 



In addition to these structures ossifications occur in the connective tissue of the maxillary 

 process, a structure which may be regarded as forming the anterior part of the first branchial 

 arch, and in the fronto-nasal process. The ossifications in the maxillary process give rise to 

 the pterygoid (medial pterygoid process of the sphenoid), the palate, the maxilla, and the 

 zygomatic, while that in the fronto-nasal process forms the premaxilla. 



The bony elements of the head may therefore be arranged, according to their origin, in the 

 foUowing table: — 



I. Basilar Bones Developed in the Cartilaginous Cranium 



Basi-occipital Basilar portion of the occipital bone. 



Exoccipitals Condylar parts of the occipital bone. 



Supra-occipital Lower part of the squamous portion of the occipital. 



P?l-s^ hlnoM^ } Constituting the body of the sphenoid. 



Ali-sphenoids Greater wings and lateral pterygoid plates. 



Orbito-sphenoids Lesser wings. 



Petro -mastoids Petrous and mastoid portions (excepting post-auditory 



processes) of the temporal bones. 



II. Roof Bones Developed in the Membranous Cranium 



Squamosals Squamous portions of temporals. 



Parietals The two parietal bones. 



Frontals United to form a median frontal bone. 



Interparietal Upper part of squamous portion of occipital. 



Epipterics The epipteric bones. 



III. Bones of the Nasal Region 



Mesethmoid Vertical plate of ethmoid developed in the cartilage of the 



cranio-facial axis. 



Ethmo-turbinals Superior and inferior conchal processes of ethmoid. 



Maxillo-turbinals The inferior nasal conchse. 



Cribriform lamina Cribriform plate of ethmoid. 



These elements are developed in the cartilage of the lateral nasal process. 



Sphenoidal turbinals Sphenoidal conchae. These are derivatives of the ethmo- 

 turbinals. 



Lacrimals The lacrimal bones 1 Developed in the membrane over 



Nasals The nasal bones J the lateral nasal process. 



Vomer The vomer. Ossified in the membrane investing the carti- 

 lage of the cranio-facial axis. 



IV. Facial Bones 



Maxillae The maxillae 1 Developed in the connective tissue 



Zygomatics The zygomatic bones / of the maxillary process. 



Premaxillae The incisor parts of the maxillae. Formed at the anterior 



extremity of the cranio-facial axis in the tissue of the 

 fronto-nasal process (proc. globulares). 



