The =base forms= the outer boundary of the foramen lacerum basis cranii. It is very irregular and presents a number of important features. The =hyoid process= is a short rod which projects downward and forward below the base of the external auditory process, inclosed in a bony tube; it is connected by a bar of cartilage with the hyoid bone. The =stylo-mastoid foramen= is situated between the root of the hyoid process and the mastoid process; it is the external opening of the facial canal, through which the facial nerve emerges. The =bulla ossea= is a considerable eminence situated centrally; it is thin-walled and incloses a cavity which is part of the tympanum. The =muscular process=[14] is a sharp spine which projects downward and forward from the anterior part of the base; it gives origin to the tensor and levator palati muscles. External to the root of the preceding is the small =petro-tympanic= or =Glaserian fissure= (Fissura petro-tympanica) for the passage of the chorda tympani nerve. The osseous =Eustachian tube= is a semicanal at the inner side of the root of the muscular process; it leads to the tympanum. At the inner side of the preceding is the slit-like orifice of the petrosal canal, which communicates with the facial canal. The =apex= projects upward and backward between the squamous temporal and the occipital bone. =Development.=—The petrous temporal may be regarded as consisting of petro-mastoid and tympanic parts. The latter includes the external auditory process, the bulla ossea, and the muscular process; it is developed in membrane. The petro-mastoid is developed in the cartilaginous ear capsule. Its petrous part consists of very dense bone which contains the labyrinth or internal ear and forms the inner wall of the tympanum. The =parieto-temporal canal= (Meatus temporalis) is a continuation of the transverse groove which extends outward from the base of the tentorium osseum. It is directed downward, forward, and somewhat outward, and opens externally in front of the root of the auditory process. It is bounded by the squamous temporal externally, the petrous behind, and the parietal in front and internally. Several foramina open from it into the temporal fossa. It contains a large vein (Vena cerebralis dorsalis), the continuation of the transverse sinus of the dura mater. The =foramen lacerum basis cranii= (Foramen lacerum et jugulare) is a large irregular opening in the cranial base, bounded internally by the basilar part of the occipital bone, externally by the petrous temporal, and in front by the temporal wing of the sphenoid. It consists of a large anterior part (Foramen lacerum anterius), and a narrow posterior part (Foramen lacerum posterius s. jugulare). It transmits the internal carotid artery, the middle meningeal artery, the mandibular, ninth, tenth, and eleventh cranial nerves, and the inferior cerebral vein.