duct (Rathke's pouch from the stomodaeum) and medial to 

 the internal carotid arteries, are the paired, hypophyseal 

 cartilages connecting with the trabecula communis ante- 

 riorly. Anterior and lateral to the internal carotid arteries 

 are paired, polar cartilages, each laterally giving rise to a 

 processus alaris. This process extends posteriorly, lateral to 

 the internal carotid artery, to form the alicochlear commis- 

 sure with the capsule. This commissure later breaks down. 

 The polar cartilages fuse with the trabecula anteriorly and 

 with the hypophyseal cartilages medially. The latter fijse 

 medially, forming the hypophyseal plate and obliterating 

 the hypophyseal foramen, and posteriorly with the basal 

 plate. The ala temporalis appears lateral to the processus 

 alaris and almost immediately attaches to its tip. This 

 chondrification comes to enclose the maxillary branch of the 

 trigeminal nerve, thus forming the foramen rotundum. 



Further forward, lateral to the trabecula communis, an 

 orbital cartilage appears dorsolateral to the optic nerve. 

 Then a center appears ventral to the nerve, the ala hypo- 

 chiasmatica. The orbital cartilage extends medially below 

 the nerve to connect with the trabecula and to the ala 

 hypochiasmatica, which also fuses medially with the trabec- 

 ula. The dorsal end of the orbital cartilage extends medially 

 and down to connect with the ala hypochiasmatica and the 

 trabeculae completing encirclement of the optic foramen. 

 The first (posterior) of these connections is the pila metop- 

 tica, the latter the pila preoptica. 



To this point, the trabecula communis has extended for- 

 ward and upward as the interorbital and nasal septum. 

 The paranasal cartilages have appeared covering the lateral 

 and posterior aspects of the nasal passages. Anteriorly the 

 paranasal cartilages fuse with the dorsal margin of the nasal 

 septum. The paraseptal cartilages have appeared and estab- 

 hshed contact with the nasal septum through the anterior 

 transverse laminae. The final steps include formation of the 

 cribriform plate by medially directed strands from the nasal 

 capsule margin and the tips of the occipital arches meeting 

 behind or above the spinal cord to form the tectum poste- 

 rior enclosing the foramen magnum. 



The chondrocranium of the cat is much like that of man, 

 differing only in details. Bilateral supraoccipital plates arise, 

 separated from the rest of the occipital arch, and join the 

 arch to the otic capsules. The tectum posterior is a connect- 

 ing growth between the supraoccipital plates. Above the otic 

 capsule a separate parietal plate arises and extends forward 

 as the orbitoparietal commissure connecting with the orbital 

 cartilage. The ala temporalis arises well before the appear- 

 ance of the processus alaris and does not fuse with that 

 structure when they come in contact. The hypophyseal car- 

 tilages appear as a single crescentic mass behind the hy- 

 pophyseal duct. The polar cartilages are indistinct, arising 

 in contact with the hypophyseal cartilage and primarily 

 forming the alicochlear commissures. The dorsal margin of 

 the nasal septum is extended laterally as the tectum nasi; 

 its margin is indicated by the epiphaniale foramen. The 



orbitonasal lamina of the nasal capsule arises separately, 

 later fusing with the paranasal cartilage and with the mid- 

 line septum just posterior to the cribriform area. The crista 

 galli is inconspicuous. 



The definitive chondrocranium of the cat shows certain 

 differences which can be summarized as follows: (1) A pos- 

 terior basicochlear fissure is present between the otic cap- 

 sule and the basis cranii, anterior to the jugular foramen. (2) 

 There is a large parietocapsular fissure in place of the small 

 mastoid foramen. (3) There are large parietal plates con- 

 nected with the otic capsules in front of the parietocapsular 

 fissure (or fissures), the supraoccipital cartilage behind and 

 the orbital cartilage in front. (4) The lamina orbitonasalis 

 is fused with the interorbital septum and connected by a 

 commissure with the pila preoptica. (5) Two pairs of 

 epiphaniale foramina are present. (6) Anterior and posterior 

 transverse laminae connectives join the nasal septum and 

 the lateral capsular wall in front of and behind the fenestra 

 choanae. (7) The paraseptal cartilages are connected to the 

 posterior transverse laminae rather than to the anterior one. 

 The rabbit diff"ers (Figure 3-14) from the cat in that it 

 lacks a zona annularis; the anterior transverse lamina does 

 not extend across from the paraseptal cartilage to the nasal 

 septum; the paraseptal cartilages are joined anteriorly and 

 posteriorly to the incomplete transverse laminae. In the cat 

 the paraseptal cartilages are joined to the medially incom- 

 plete posterior transverse lamina and are free anteriorly. The 

 cat has a posterior basicochlear fissure rather than an ante- 

 rior basicochlear fissure characteristic of the rabbit, which 

 has an occipitocapsular fissure, lacking in the cat. The cat 

 has a single hypoglossal foramen through which pass three 

 roots, whereas the rabbit has two pairs of foramina. The 

 rabbit has a larger tegmen tympani, which fuses sooner with 

 the cochlear part of the capsule to separate the foramen 

 primitivum from the hiatus facialis. The ala temporalis of 

 the cat has a foramen rotundum; that of the rabbit lies be- 

 tween the maxillaris and mandibular roots and is penetrated 

 by an alar canal. Meckel's cartilage of the cat is fused at the 

 tip. 



Marsupial The chondrocranium of the opossum is much 

 like that of some of the placentals, but it diff'ers in having a 

 wider orbitoparietal commissure; two hypoglossal foramina; 

 a dorsal nasal tegmen, which grows out from the dorsal 

 margin of the nasal septem and forms an anterior cupola; a 

 wide zona annularis, formed by the anterior transverse 

 lamina; and a paraseptal cartilage with a parallel lateral 

 process. It also differs in lacking a parietocapsular fissure, 

 a tegmen tympani subdividing the outer opening of the 

 facial canal, an ala temporalis, a pila metoptica, and a lam- 

 ina transversus posterior to the nasal capsule; and in hav- 

 ing the Meckel's cartilages fused anteriorly. 



The opossum diff'ers from other marsupials in that: A 

 small parietocapsular foramen may be present. A cartilagi- 

 nous ala temporalis is usually present which may or may not 



54 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



