314 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



to form a single center from which the whole cartilage is transformed into the 

 petrous and mastoid portions of the temporal bone (Fig. 320). The mastoid 

 process is formed after birth by a bulging of the petrous bone, and its internal cavi- 

 ties, the mastoid cells, are formed and lined by the evaginated epithelial Hning of 

 the middle ear. The squamosal and tympanic portions of the temporal bone are 

 of intramembranous origin, while the styloid process originates from the proximal 

 end of the second, or hyoid, branchial arch. 



Membrane Bones of the Skull.— From the preceding account it is evident 

 that although the bones forming the base of the skull arise chiefly in cartilage, they 

 receive substantial contributions from membrane bones. The remainder of the 

 sides and roof of the skull is whoUy of intramembranous origin, each of the parietals 



Malh IIS 



Slopes 

 Styloid process' 



Tympanic rin 

 Stylo-kyoid lit^ 



Mandible 



Cricoid cartilage 



Meckel's cartilage 



Hyoid cartilage {lesser horn) 

 Hyoid cartilage (greater horn) 



TJiyreoid cartilage 



Fig. 321. — Lateral dissection of the head of a human fetus, showing the derivatives of the brandiial 



arches (after Kollmann). 



forming from a single center, the frontal from paired centers. At the incomplete 

 angles between the parietals and their adjacent bones union is delayed for some 

 time after birth. These membrane-covered spaces constitute the fontandles. 



The vomer forms from two centers in the connective tissue flanking the lower 

 border of the lamina perpendicularis of the ethmoid. The cartilage of the eth- 

 moid thus invested undergoes resorption. 



Single centers of ossification in the mesenchyme of the facial region give rise 

 to the nasal, lacrimal, and zygomatic, all pure membrane bones. 



Branchial Arch Skeleton. — The first branchial arch forks into an upper maxil- 

 lary and a lower mandibular process (Fig. 119). Cartilage fails to appear in the 

 maxillary processes, due to accelerated development, hence the palate bones and 



