196 TEXT-BOOK OF EMBRYOLOGY. 



formed after birth by an evagination from the pars petrosa, and is lined by an 

 evaginated portion of the mucosa of the middle ear. The other parts of the 

 temporal bone are of intramembranous origin, except the styloid process which 

 represents the proximal end of the second branchial arch. 



In the ethmoidal region, conditions become more complicated on account of 

 the peculiarities of the nasal cavities, and on account of the fact that the cartilage 

 is never entirely replaced by bone, and that "membrane" bones also enter into 

 more intimate relations with the "cartilage" bones. The ethmoidal cartilage 

 at first consists of a medial mass, which extends from the presphenoid region to 

 the end of the nasal process, and of a lateral mass on each side, which is situated 

 lateral to the nasal pit (Fig. 174). Ossification in the lateral mass on each side 

 produces the ethmoidal labyrinth (lateral mass of ethmoid). It is perhaps not 

 quite correct to say that ossification produces the ethmoidal labyrinth, for at 

 first there is only a mass of spongy bone with no indication of the honey-combed 

 structure characteristic of the adult. The latter condition is produced by a 

 certain amount of dissolution of the bone and the growth of the nasal mucosa 

 into the cavities so formed. By the same process of dissolution and ingrowth of 

 nasal mucosa the superior, middle and inferior conchce (turbinated bones) are 

 formed. The medial mass of cartilage begins to ossify after birth and then only 

 in its upper (superior) edge. It forms the lamina perpendicular is and crista 

 galli and extends into the nose as the nasal septum. The lower (inferior) edge 

 remains as cartilage until the vomer, which is a membrane bone (p. 198), 

 develops, after which it is partly dissolved. The lamina cribrosa (cribriform 

 plate) is formed by bony trabecular which extend across between the medial 

 mass and the lateral masses and surround the bundles of fibers of the olfactory 

 nerve. 



Membrane Bones of the Skull. 



Under this head we shall consider only those bones which develop apart 

 from the visceral arches, those which involve the arches being considered later. 

 It has been seen that by far the greater parts of the bones forming the base of the 

 skull are of intracartilaginous origin. On the other hand, those forming the 

 sides and roof of the skull are largely of intramembranous origin. In the case 

 of the occipital bone, two centers of ossification appear in the membrane dorsal 

 to the supraoccipital, and the bone so formed begins to unite with the supra- 

 occipital during the third month of fcetal life. At birth the union is usually 

 complete, though for a time an open suture may persist on each side. The bone 

 derived from the two centers forms that part of the occipital squama which is 

 situated above the superior nuchal line; the part below the line is of intracarti- 

 laginous origin (p. 194). The adult occipital is thus a composite bone, partly 

 of intramembranous, partly of intracartilaginous origin. 



