THE BONES OF THE HEAD 165 



Vomer. — This bone is developed in the mesoderm, which nvests the 

 ■posterior and inferior parts of the septal cartilage of the nose, that cartilage 

 being derived from the ethmoidal division of the basal sheet of cartilage or 

 chondrocranium . 



Styloid Process of Temporal Bone. — Both parts of this process — namely, the 

 tympano-hyal and st^^lo-hj-al — are developed from the proximal or dorsal 

 end of the hyoid bar (Reichert's cartilage) of the second visceral or hyoid arch. 



Summary. — The first visceral or mandibular arches contribute the following 

 bones to the facial portion of the visceral-arch skeleton: (i) The greater part 

 of each superior maxilla; (2) the greater part of the inferior maxilla or man- 

 dible; (3) the two malar bones; and the two palate bones. 



The fronto-nasal process and the ethmoidal division of the basal cartilage, 

 or chondrocranium, contribute the following bones : (i) Pre maxillary portion 

 of each superior maxilla; (2) the two nasal bones; (3) the two lachrymal bones; 

 and (4) the two inferior turbinate bones. 



The vomer, as just stated, belongs to the mesodermic investment of the 

 posterior and inferior part of the septcd cartilage of the nose, which cartilage 

 is derived from the ethmoidal division of the chondrocranium. 



The greater part of the hard palate is derived from the palatal shelves of 

 the two maxillary processes of the mandibular arches, but the premaxillary 

 part is formed by the nasal laminae of the globular processes of the mesial 

 nasal process of the fronto-nasal process. 



The Hyoid Bone. 



The hyoid bone forms a part of the visceral-arch skeleton of the skull, and 

 the structures concerned in its development are the second and third visceral 

 arches of each side. 



Second Visceral or Hyoid Arch. — ^The cartilage of this arch is known as 

 the hyoid bar, or cartilage of Reichert, and its distal or ventral segment gives 

 rise to the small cornu or cerato-hyal. 



Third Visceral or Thyro-hyoid Arch. — ^The distal or ventral end of the 

 cartilage of this arch is connected with that of its fellow of the opposite side 

 by a median piece of cartilage, which acts as a bond of union between them, 

 and is from that circumstance called the copula. The copula also connects 

 the right and left hyoid bars. The body of the hyoid bone, or the basi-hyal, 

 is developed from the copula; and each great cornu^ or thyro-hyal, is developed 

 from the cartilaginous bar of the corresponding third visceral arch. 



The bones of the visceral-arch skeleton of the skull, genetically considered, 

 are (i) membranous, (2) complex or mixed — that is to say, partly membranous 

 and partly cartilaginous — and (3) cartilaginous. 



Membrane Bones : Complex or Mixed Bone : 

 Superior Maxillae. Inferior Maxilla. 



Malars. Cartilage Bones : 

 Nasals. Inferior Turbinates. 



Lachrymals. Hyoid Bone. 



Palate Bones. 

 Vomer. 



Osseous Stage. — The osseous stage in the development of the skull is treated 

 under each individual bone. 



THE TEETH. 



The teeth are divided into two sets, namely, the temporary, milk, 

 or deciduous, which belong to early infancy, and the permanent, 

 which replace the temporary. The temporary teeth are twenty in 

 number — ten upper, five in each superior maxilla, and ten lower, 

 five in each half of the inferior maxilla. The number of permanent 

 teeth is thirty-two — sixteen upper, and sixteen lower. 



