THE INTERIOR OF THE CRANIUM. 71 



The occipital bone, the petrous and mastoid portions of the temporal, the postero- 

 inferior angle of the parietal, and the body of the sphenoid take part in its formation. 

 In the centre of the fossa is the foramen magnum ; and on each side of this, in 

 a nearly vertical line from below upwards, are the anterior condylar foramen piercing 

 the condylar portion of the occipital bone, the jugular foramen between the occipital 

 and petrous, and the internal auditory meatus on the posterior surface of the petrous. 

 Behind the jugular foramen is the posterior condylar foramen (if present), and higher 

 up the more constant mastoid foramen, both opening into the groove of the lateral 

 sinus. 



The foramen magnum is occupied in the recent state by the lower end of the medulla 

 oblongata with its membranes, the vertebral arteries, and the spinal accessory nerves. The 

 anterior condylar foramen transmits the hypoglossal nerve and a meningeal branch of the 

 ascending pharyngeal artery. By the internal auditory meatus the facial and auditory nerves, 

 with the portio intermedia and the auditory vessels, leave the cranial cavity. The posterior 

 condylar and mastoid foramina transmit emissary veins, the latter also the mastoid branch of 

 the occipital artery. 



The jugular foramen (foramen lacerum posterius) is formed by the jugular notches of the 

 petrous and occipital bones : somewhat pyriform in shape, two more or less marked constric- 

 tions indicate a division into three compartments ; most externally and posteriorly is a large 

 rounded part in which the lateral sinus joins the internal jugular vein ; the middle part, cor- 

 responding to a distinct notch in the lower border of the petrous, transmits the glosso- 

 pharyngeal, vagus, and spinal accessory nerves ; and the most anterior and internal, sometimes 

 completely separated by a spiculum of bone (the intra jugular process; p. 34), gives passage to 

 the inferior petrosal sinus. A meningeal branch of the ascending pharyngeal or occipital 

 artery also enters the skull by the posterior compartment. 



Grooves for blood-vessels. The groove of the middle meningeal artery 

 commences at the foramen spinosum, and ramifies principally on the squamous 

 portion of the temporal bone and on the parietal. The groove of the internal 

 carotid artery lies on the side of the body of the sphenoid, and extends from 

 the foramen lacerum to the inner side of the anterior clinoid process. The groove of 

 the superior longitudinal sinus, commencing at the frontal crest, passes backwards in 

 the middle line of the roof of the skull, and terminates at the internal occipital 

 protuberance. From that point the groove of the lateral sinus passes outwards on 

 each side over the occipital bone, crosses the posterior inferior angle of the parietal 

 bone, descends on the mastoid portion of the temporal bone, runs inwards again on 

 the occipital, and turns forwards to terminate at the jugular foramen. The groove 

 of the inferior petrosal sinus lies between the petrous portion of the temporal bone 

 and the basilar process ; that of the superior petrosal sinus extends along the superior 

 edge of the petrous portion. 



EHE NASAL CAVITIES AND COMMUNICATING AIR-SINUSES. 



The nasal cavities, or fossae, are placed one on each side of a median vertical 

 septum. They open in front by the anterior nasal aperture and behind by the 

 posterior nares already described, and communicate with the sinuses of the frontal, 

 ethmoid, sphenoid, and superior maxillary bones. Their vertical extent, as well as 

 that from before backwards, is considerable, but their transverse width is very 

 limited, especially in the upper part. 



The internal wall, or septum nasi, is formed by the central plate of the ethmoid, 

 the vomer, the nasal spine of the frontal, the rostrum of the sphenoid, and the crests 

 of the nasal, maxillary, and palate bones. It presents a great angular deficiency in 

 front, which in the recent state is filled up by the septal cartilage. In most cases it 

 deviates somewhat from the middle line to one side or the other. 



The roof is horizontal in its middle part, but sloped downwards before and 

 behind. The middle part is formed by the cribriform plate of the ethmoid, the fore 



