with the basisphenoid in man to form the os sphenoidale. 



The basi pterygoid is usually represented by paired centers 

 but sometimes there is only a single median center (many 

 insectivores) or two pairs of centers (man). The more lateral 

 pair of centers in man, the lingulae, appear in the alicoch- 

 lear commissures. 



The pterygoid is also thought to be a compound bone in 

 its origin, having two centers, one above the other. One cen- 

 ter encloses the pterygoid cartilage, the hamulus, and the 

 other forms the plate connecting this process to the base of 

 the cranium. Two pterygoid appear to be two parted in the 

 rat and in some other mammals (flying lemur, etc.), but usu- 

 ally only a single ossification center is known to occur. In man 

 or the cat, ossification begins in the dermal base of the 

 pterygoid and spreads to the hamular process. 



Ossification of the chondral parts of the ethmoid region, 

 the interorbital septum, orbitonasal lamina, cribriform plate 

 and crista galli, and nasal capsule is late and irregular. 

 Separate centers appear in each of these subdivisions and 

 eventually fuse to form a bony continuum. 



The visceral skeleton presents points of interest — the ala 

 temporalis has already been discussed as a part of the 

 alisphenoid. The mandible is made up of the dentary only. 

 The tympanic appears early as a semicircle of bone, and 

 above its anterolateral free end a small dermal prearticular 

 (or supraangular) appears which later fuses with the endo- 

 chondral ossification of the posterior end of Meckel's carti- 

 lage, the malleus. In man (Figure 3-4), this anterior process 

 is caught between the petrosal and tympanic, and the en- 

 trapped part becomes ligamentous, attaching the malleus 

 to the tympanic wall. The incus begins to ossify after the 

 malleus and the stapes ossifies late in development. Except 

 for the basihyal, ossification in the hyoid is late, and there 

 is much variation in the extent of ossification. 



Marsupial Comparisons of the opossum with the placental 

 indicates close agreement. The opossum differs in lacking 

 an ala temporalis; the alisphenoid is entirely dermal in orjgin 

 but extends outward from the tip of the processus alaris 

 much as it would if the ala were present. This dermal anlage 

 encloses the foramen rotundum instead of lying between that 

 foramen and the foramen ovale. Lack of an ala temporalis 

 is not typical of marsupials, most of which agree with the 

 placental in having this structure. The pterygoid stems from 

 a single center and has only a small cartilaginous, or a pro- 

 cartilaginous, precursor for the hamulus. The presphenoid 

 centers of the orbitosphenoid appear first and fuse at the 

 midline. To this, the splint-like preoptic pillar center attaches 

 when it appears. There is no posterolateral or lateral center 

 of ossification; the main one of most placentals, as a result 

 the optic foramen and orbital fissure, are confluent. The 

 nature of the pterygoid and orbitosphenoid can be matched 

 also in placentals. A small midline spur of bone, the para- 

 sphenoid, becomes attached to the ventral aspect of the 

 basisphenoid near its anterior margin. The usual absence of 



palatine 

 presphenoid center of 



orbitosphenoid 

 alisphenoid 



pterygoid 

 prearticular 



a ^basisphenoid 

 bosioccipital 



A JUST BEFORE BIRTH 



orbitosphenoid 



palatine 



alisphenoid 

 foramen rotundum 

 foramen ovale 



basisphenoid 



prearticular 

 alleus 



B 



orbitosphenoid 



palatine 

 alisphenoid 



foramen ovale 



processus alaris 

 /'"^^^^^^ pterygoid 



bosioccipital 



5 DAYS 



Figure 3-1 7. Ossifications in the basis cranii of the rat at three stages 

 of growth as seen fronn above. A, just before birth; B, one day old; 

 C, five days old. 



DEVELOPMENT OF THE HEAD SKELETON • 59 



