THE MORPHOLOGY OF THE SKULL 125 



junction of the occipital with the petrosal and sphenoid. The dorsum sellae is entirely carti- 

 laginous at birth, and the last portion of this cartilage disappears with the ankylosis of the 

 basi-occipital and basi-sphenoid about the twentieth year. A strip of cartilage unites the aU- 

 sphenoids with the lingulae, and for at least a year after birth this cartilage is continuous with 

 that which throughout life occupies the foramen lacerum. A strip of cartilage exists along the 

 posterior border of the orbito-sphenoid, and not unfrequently extends lateralward to the 

 pterion. In the adult skull it is replaced by ligamentous tissue. 



The ethmo-vomerine plate is entirely cartilaginous, and near the end of the nose supports 

 the lateral nasal cartilages, remnants of the fronto-nasal plate. The fate of the ethmo-vomerine 

 plate is instructive. The upper part is ossified to form the mesethmoid; the lower part atro- 

 phies from the pressure exerted by the vomer; the anterior end remains as the septal cartilage. 

 The lateral snout-like extremities of the fronto-nasal plate persist as the lateral cartilages of the 

 nose. 



Among the appendicular elements of the skull, the styloid process and a large portion of the 

 hyoid are cartilaginous at birth. 



The Nerve-foramina of the Skull 



The various foramina and canals in the skull which give passage to nerves may be arranged 

 in two groups, primary and secondary. Primary foramina indicate the spots where the nerves 

 leave the general cavity of the dura mater, and as this membrane indicates the limit of the 

 primitive cranium, a cranial nerve, in a morphological sense, becomes extra-cranial at the point 

 where it pierces this membrane. In consequence of the complicated and extraordinary modifica- 

 tions the vertebrate skull has undergone, many nerves traverse, in the adult skull, bony tunnels 

 and canals which are not represented in the less complex skulls of low vertebrates, such as 

 sharks and rays. To such foramina and canals the terms secondary or adventitious may be 

 applied. 



Nerve-foramina are further interesting in that they occupy sutures, or indicate the points 

 of union of two or more ossific centres. To this rule the foramen rotundum is the only excep- 

 tion in the human skull. 



The Primary Foramina 



1. Foramen magnum. — This is bounded by four distinct centres, the supra-, basi-, and 

 two ex-occipitals. It transmits the accessory (eleventh) pair of cranial nerves, the vertebral 

 arteries and their anterior and posterior spinal branches, the medulla oblongata and its mem- 

 branes, and the membrana tectoria. 



2. The hypoglossal. — At bu-th this is a deep notch in the anterior extremity of the ex- 

 occipital, and becomes a complete foramen when the basi- and ex-occipitals fuse. Occasionally 

 it may be complete in the ex-occipital, but it indicates accurately the line of union of these two 

 elements of the occipital bone. It transmits the hypoglossal nerve, the meningeal branch of the 

 ascending pharyngeal artery, and its venae comitantes. 



3. Jugular foramen. — This occupies the petro-occipital suture, and is formed by the basi- 

 and ex-occipital in conjunction with the petrosal. It transmits the glosso-pharyngeal, vagus, 

 and accessory nerves, a meningeal branch of the ascending pharyngeal artery, and receives the 

 transverse and inferior petrosal sinuses. 



4. Auditory. — This marks the point of confluence of the groups of centres termed pro-otic 

 and opisthotic. It transmits the facial and auditory nerves, the pars intermedia, and the au- 

 ditory twig of the basilar artery. 



5. Trigeminal. — This is only a foramen when the dura mater is present in the skull. It 

 is a notch at the apex of the petrosal converted into a foramen by the tentorium. The main 

 trunk of the trigeminal nerve, with the small motor root (masticator nerve), traverses it. 



6. Petro-sphenoidal. — -This is a notch between the side of the dorsum sellae and apex of 

 the petrosal which becomes converted into a foramen by dura mater. 



7. Optic. — This foramen is formed by the confluence of the orbito- and pre-sphenoidal 

 centres. It opens into the orbit and transmits the optic nerve and ophthalmic artery. 



The Secondary Nerve -foramina 



Foramina transmitting the various subdivisions of the trigeminal nerve. — The primary 

 foramen of exit for the trigeminal nerve is formed partly of bone and partly of membrane at 

 the apex of the petrosal. The three divisions of the nerve issue throu2:h secondary foramina. 



(a) The superior orbital (sphenoidal) fissure is an elongated chink, bounded above by 

 the orbital wing and below by the great wing of the sphenoid, medially by the body of the 

 sphenoid, and laterally by the frontal. It opens into the orbit, and transmits the third, fourth, 

 first (ophthalmic) division of the trigeminal and abducens nerves, also the ophthalmic vein or 

 veins. 



(6) The foramen rotundum is the only exception to the rule relating to the formation of 

 nerve-foramina; it is probably a segment of the superior orbital fissure. The foramen is really 

 a canal running from the middle cranial fossa to the pterygo-palatine fossa, and transmits the 

 second or maxillary division of the trigeminal. 



(c) The foramen ovale at birth is a gap in the hinder border of the great wing (ali-sphe- 

 noid) of the sphenoid, and is converted into a foramen by the petrosal; subsequently it becomes 

 complete in the sphenoid. It transmits the third or mandibular division of the trigeminal and 

 the small or motor root, the small superficial petrosal nerve (which occasionally passes through 

 a separate foramen), and the small meningeal artery with its vense comitantes. 



