38 BONES OF THE HEAD. 



emerges at the apex, close to the anterior margin. It transmits the internal 

 carotid artery. In the angle between the petrous and squamous portions 

 of the bone is the anterior opening of the Eustachian canal. 



The posterior surface looks backwards and inwards, and forms part of 

 the posterior fossa in the base of the skull. It presents a large orifice 

 leading into a short canal which is directed outwards, the meatus auditorius 

 internus. This canal is terminated by a lamella of bone, the lamina cribrosa, 

 presenting in the lower part small apertures through which the fibrils of the 

 auditory nerve pass, while in its upper part is the commencement of the 

 caual called aqueduct of Fallopius, which transmits the facial nerve. The 

 facial canal takes a somewhat circuitous course through the petrous bone, 

 passing outwards and backwards over the labyrinth of the ear, and then 

 downwards to terminate at the stylo-mastoid foramen. 



The anterior or upper surface looks upwards and forwards, and forms 

 part of the middle fossa in the base of the skull. A depression over the 

 apex marks the position of the Gasserian ganglion. A narrow groove runs 

 obliquely backwards and outwards to a foramen named the hiatus Fallopii, 

 which leads to the aqueduct of Fallopius ; and lodges the large superficial 

 petrosal nerve. Farther back is a rounded eminence, indicating the situa- 

 tion of the superior semicircular canal. 



The superior border is grooved for the superior petrosal sinus. The 

 anterior border is very short, and forms at its junction with the squamous 

 part an angle in which is situated the orifice of the Eustachian canal, the 

 osseous portion of a tube of the same name, which leads from the pharynx 

 to the tympanum ; and above this, partially separated from it by a thin 

 lamella, the processus cochleariformis, a small passage which lodges the tensor 

 tympani muscle. The posterior border articulates with the basilar process 

 of the occipital bone, and bounds the foramen lacerum posterius. 



Small foramina. The opening of the aqueductus vestibuli is a narrow fissure, 

 covered by a depressed scale of bone, and situated on the posterior surface of the 

 petrous bone, about three lines behind the internal auditory meatus; that of the 

 aqueductus cochleae, is a small foramen, beginning in a three-sided wider depression 

 in the posterior margin, directly below the internal auditory meatus. In the plate 

 between the jugular fossa and the carotid canal is the foramen by which the nerve of 

 Jacobson passes to the tympanum. In the ascending part of the carotid canal is the 

 minute foramen for the tympanic branch of the carotid plexus. In the jugular fossa 

 are a groove and foramen for the auricular branch of the vagus nerve ; and parallel 

 to the hiatus Fallopii, close to the canal for the tensor tympani muscle, are a groove 

 and foramen for the small superficial petrosal nerve. 



THE SPHENOID BONE. 



The sphenoid, or wedge-shaped bone, is placed across the base of the skull, 

 near its middle. It enters into the formation of the cavity of the cranium, 

 the orbits, and the posterior nares. It is of very irregular shape, and consists 

 of a central part or body, a pair of lateral expansions called the great wings, 

 which form the largest part of the bone, a pair of much smaller horizontal 

 processes in front, called the small wings, and a pair which project down- 

 wards, the pterygoid processes. 



The sphenoid is articulated with all the seven other bones of the cranium 

 and with five of those of the face, viz., posteriorly with the occipital and 

 with the petrous portions of the temporals, anteriorly with the ethmoid, 

 frontal, and malars, laterally with the squamous portion of the temporals, 

 the parietals and frontals, and inferiorly with the vomer and palate bones. 



