Anterodorsal to the basitrabecular process is the pila 

 metoptica ("pillar behind the optic nerve") of either side. 

 The gap between these is the orbital fissure. The pila me- 

 toptica lies behind the optic foramen and the pila preoptica 

 forms the anterior margin of that opening. Both pilaejoin 

 the ala orbitalis or orbital wing laterally. Separating the 

 pila preoptica from the nasal capsule is the orbitonasal fis- 

 sure; the orbital wing is connected lateral to this fissure to 

 the nasal capsule by the sphenethmoid commissure. The 

 sphenethmoid cominissure and orbital wing lie in the upper 

 and medial walls of the orbit of the eye. 



Extending upward from the basis cranii (cranial base), 

 in front of the optic foramina, is a midline vertical plate of 

 cartilage identified as the interorbital septum. This septum 

 continues anteriorly as the nasal septum. 



The nasal capsules are formed by lateral cup-like masses 

 (paranasal cartilages), fused above with the dorsal margin 

 of the nasal septum; posteriorly they contact the nasal sep- 

 tum but do not fuse with it. The dorsoposterior gap between 

 these cups and the nasal septum are closed by a fenestrated, 

 cartilaginous, cribriform plate. Above this rises the dorsal 

 margin of the nasal septum, the crista galli. Lateral to the 

 cribriform area is the attachment of the sphenethmoid com- 

 missure. Anterior to the end of this commissure is the fora- 

 men epiphaniale. 



Posteroventrally there is a small connection, the lamina 

 transversalis posterior, between the cup and the nasal sep- 

 tum. Anterior to this connection there is no contact; the 

 fenestra narina is continuous with the fenestra choanae. 

 The internal walls of the nasal capsules have three medially 

 projecting shelves giving rise to the turbinals. Near the 

 anterior end of the nasal septum there is a small lateral pro- 

 jection from the ventral margin, the anterior transverse 

 lamina (lamina transversalis anterior). From the tip of it 

 rises a backward projecting spur, the paraseptal cartilage, 

 associated with the Jacobson's organ. The anterior trans- 

 verse lamina does not reach the lateral wall of the capsule 

 to complete a zona annularis (Figure 3-15). 



The visceral skeleton is represented by several cartilagi- 

 nous structures. The ala temporalis has already been de- 

 scribed. The quadrate cartilage is transformed into the incus. 

 The lower jaw is represented by Meckel's cartilage, which 

 posteriorly is much expanded with a ventromedially pro- 

 jecting manubrial process. This posterior part gives rise to 

 much of the malleus. 



The hyoid arch consists of the cartilaginous stapes and 

 Reichert's cartilage from the posterior margin of the paroc- 

 cipital process margining the tympanic fossa, and also a 

 body and a small nodule of ceratohyal. 



The first branchial arch is represented by a large cerato- 

 branchial I connected with the thyroid cartilage. The 

 thyroid cartilage represents a fusion product of the cerato- 

 branchials II and III. 



The earlier stages of development of the chondrocranium 

 reveal the origins of the various parts in terms of centers of 



chondrification (Figure 3-13). Cartilage is formed in a mes- 

 enchymatous blastema (condensation) and is preceded bv a 

 procartilaginous stage. Later as the cartilage grows and 

 expands into new areas, a more direct conversion of mesen- 

 chyme cells, from the inner aspect of the enclosing peri- 

 chondrium, takes place. 



The first cartilages to appear are the paired parachordals 

 lying at either side of the anterior end of the notochord. 

 These develop a connection at their anterior ends, above 

 the notochord, called the crista transversa. Posteriorly the 

 parachordals are continuous with two pairs of occipital 

 arches, between which pass the hypoglossal roots. The more 

 anterior pair of these arches seems to be related to the 

 parietal plates appearing above the otic capsules. The two 

 arches of a side fuse for most of their distal length, isolating 

 the hypoglossal foramen between their bases and next to the 

 parachordal. 



Lateral to the parachordals are the free otic capsules, 

 and well anterior to the ends of the parachordals there is a 

 midline trabecula communis (formed by fusion of paired 

 trabeculae). 



Following this stage, the otic capsules fuse with the parie- 

 tal plate, then with the basal plate formed by anterior-to- 

 posterior fusion of the parachordals above and below the 

 notochord. Above the position of the original crista transversa 

 an acrochordal chondrification appears which forms the 

 dorsum sellae and soon fuses to the crista transversa. Ante- 

 rior to the dorsum sellae, at either side of the hypophyseal 



nasal septum and tectum nasi 

 >-->/ ^-^ (dashed outline) 



paranasal 

 paraseptal 



orbitonasal lomina 

 trabecule communis 



Q 



/ \ ^~"^^---.h/pochiasmatic 

 O^^^ / /\\ ^^^5^°'° temporalis 



internal carotid-{-/o(J {l^i2~ — -polor 



^ — ^ "~::=:;r— hypophyseal 



acrochordal 

 otic capsule 



parachordal 

 parietal plate 



synotic tectum 



supraoccipitol 



plate 



occipital arches 



tectum posterior 



Figure 3-13. Diagram showing relative positions of the various 

 chondrification centers. 



DEVELOPMENT OF THE HEAD SKELETON • 53 



