()(3 THE SKKLKTOX 



ocrui.ios tlio pplicuotic (niiilille laccruteil) fuiaiuen. A strip of cartilage exists along the posterior 

 Imrder of tlie orliito-spheiioid, and not unfrequently extends outwards to the pterion. In the 

 adult skull it is ri'plai-cd by liganientcms tissue. 



The etlini(i-vi>nifiine i)late is entirely cartilagiiiuus, and near the end of the nose supports 

 the lateral nasal cartilages, renniants of the fronto-nasal plate. The fate of the etlnno-vouierine 

 iilate is instructive. The ui)per ])art is ossified to fonu the mesethmoid ; the lower jKirt atrophies 

 frouj the pressure exerted l)y the vomer ; the tip remains as the triangular cartilage. The 

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



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

 hyoid are cartilaginous at birth. 



The Newe-foramina of the Skull 



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

 in two f^roups, primary and secondary. Primary foramina indicate the spots where the nerves 

 ([uit tlie 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 modi- 

 fications tlie vertebrate skull has undergone, many nei-ves traverse, in the adult skull, bony 

 tunnels and canals which are not represented in the less complex skulls of low vertebrates 

 such as sliarks and rays. To such foramina and canals the terms secondary or adventitious 

 may be ai)plicd. 



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 exception in 

 the human skull. 



The Primary Foramina 



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

 ex-occijiitals. It transmits the spinal accessory nerve, the vertebral artery and its anterior and 

 posterior spinal branches, the spinal cord and its membranes. 



2. The anterior condyloid. — At birth this is a deep notch in the anterior extremity of the 

 ex-occipital, and becomes a com])lete foramen when the basi- and ex-occipitals fuse. Occasionally 

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

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

 ascending pharyngeal artery, and its venas comites. 



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

 ex-occipital in conjunction with the petrosal. It transmits the glossopharyngeal, pneumogastric, 

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

 the lateral 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 with the auditory twig of the 

 basilar arteiy. 



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 traverses it to entet" the Meckelian cave. 



G. Petro-sphenoidal. — This is a notch between the sidt; of the dorsum ephippii and apex 

 of the ))etrosal which Itecoines 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 fifth nerve. — The primary foramen 

 of exit i'or the trigcininal nerve is formed partly of bone and partly of membrane at the apex of 

 the petrosal. 'I'lu; three divisions of the nerve issue through secondary foramina. 



(a) The sphenoidal fissure is an elongated chink, bouiidc(l above by the orbital wing and 

 below by the greater wing of the sphenoid, internally by the body of the sphenoid, and externally 

 by the frontal. It opens into the orbit, and transmits the third, fourth, first (ophthalmic) 

 division of the fii'tji, and sixth nerves, also the ophthalmic vein. 



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

 ncrvij-foraniina : it is i)robably a segment of the sphenoidal fissure. The foramen is really a 

 canal running from the middle cranial fossa to the spheno-niaxillaiy fossa, and transmits the 

 second or maxillary <livision of the trigeminal. 



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

 noid) of the sphcMioid, and is converted into a Ibramen by the petrosal ; subse(|uently it becomes 

 complete in the si)henoid. It transmits the third or mandibular division of the trigeminal and 

 the small or motor root, the small superficial jietrosal nerve (which occasionally jjasses through 

 a separate foramen), and the small meningeal artery with its vv.uie comites. 



