182 OSTEOLOGY. 



the floor of the lateral parts of the fossa is thin as it overlies the temporal, infra- 

 temporal, and mandibular fossae. The grooves for the lodgment of the branches oj 

 the middle meningeal artery leading from the foramen spinosum are readily seen 

 one, coursing backwards a little below the line of the squamoso-parietal suture 

 is specially well marked. Amongst other features may be noticed the depressioc 

 for the lodgment of the semilunar ganglion overlying the apex of the petroui 

 part of the temporal; behind and to the lateral side of the hiatus facialis, the 

 arcuate eminence, indicating the position of the superior semicircular canal ; anc 

 immediately anterior and slightly to the lateral side of this the tegmen tympani, 

 which roofs in the cavity of the tympanum, the thinness of which can readily be 

 demonstrated if light be allowed to fall through the external acoustic meatus. 



The posterior fossa is larger and deeper than the others. In front it is limitec 

 by a line on either side leading backwards and laterally from each posterior clinoic 

 process along the superior border of the petrous part of the temporal bone, where 

 laterally and posteriorly it becomes confluent with the superior lip of the transverse 

 groove for the transverse sinus, ending posteriorly in the middle line at the internal 

 occipital protuberance. Along the line thus indicated the process of dura matei 

 called the tentorium cerebelli, which roofs in the posterior fossa, is attached. The 

 floor of the fossa, in which the cerebellar hemispheres, the pons, and medulla oblongata 

 are lodged, is formed by the petrous and mastoid portions of the temporal bone 

 with part of the body of the sphenoid and the basilar portion of the occipital bone 

 wedged in between them. Above the mastoid part of the temporal a small part oJ 

 the mastoid angle of the parietal enters into the constitution of the side wall oi 

 the fossa. Behind and within these the lateral parts and inferior portions of the 

 squamous part of the occipital complete the floor. In the median plane the floor 

 of the fossa is pierced by the foramen magnum, in which lies the lower part of the 

 medulla oblongata, together with its membranes, and through which pass upwards 

 the vertebral arteries and the accessory nerves. On either side of the foramen 

 magnum, and a little in front of a transverse line passing through its centre, is 

 the opening of the canalis hypoglossi for the passage of the hypoglossal nerve, 

 a small meningeal branch from the ascending pharyngeal artery and an emissary 

 vein. Overhanging the opening of the canalis hypoglossi there is a thickened 

 rounded bridge of bone, to the lateral side of which is placed the irregular opening 

 of the jugular foramen. The size of this is apt to vary on the two sides, and the 

 lumen is frequently subdivided by a spicule of bone which runs across it ; the 

 posterior and lateral rounded part of the foramen is occupied by the transverse 

 sinus, which here joins the internal jugular vein. A meningeal branch from the 

 ascending pharyngeal or occipital artery also enters the skull through this com- 

 partment. The anterior and medial part of the foramen is confluent with the groove 

 for the inferior petrosal sinus, which turns downwards in front of the spicule above 

 referred to. The interval between the portions of the foramen occupied by the two 

 veins allows the transmission of the glosso-pharyngeal, vagus, and accessory nerves 

 in this order from before backwards. About a quarter of an inch above and to 

 the lateral side of the anterior part of the foramen jugulare the posterior surface of 

 the petrous portion of the temporal bone is pierced by the internal acoustic 

 meatus, through which the facial and acoustic nerves, together with the nervus 

 intermedius, and the auditory branch of the basilar artery, leave the cranial cavity. 

 Behind the jugular foramen and close to the margin of the foramen magnum 

 the opening of the canalis condyloideus, when present, may be seen. This gives 

 passage to a vein which joins the vertebral vein inferiorly. The inner aperture of the 

 mastoid foramen is noticed opening into the groove for the transverse sinus, a little 

 below the level of the superior border of the petrous part of the temporal. Through 

 it passes an emissary vein which joins the occipital vein laterally; the mastoid 

 branch of the occipital artery also enters the cranial cavity through this foramen. 



The posterior fossa is divided into two halves posteriorly by the internal occipital 

 crest, to which the falx cerebelli is attached, the floors of the hollows on either side 

 of which are often exceedingly thin and are for the lodgment of the hemispheres of 

 the cerebellum. The grooves for the following blood sinuses are usually distinct 

 the superior petrosal running along the superior border of the petrous part of the 



