THE SPHENOID BONE. 43 



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 so-called fissure of Glaser is in the inner portion of its extent a double cleft, the 

 tympanic plate being here separated from the squamous division of the bone by a descending 

 process of the tegmen tympani, which forms the greater part of the outer wall of the common 

 canal of the tensor tympani and Eustachian tube. Between this process and the tympanic 

 plate is a small orifice leading to the cavity of the tympanum, and lodging in the recent 

 state the slender process of the malleus and the tympanic branch of the internal maxillary 

 artery ; farther inwards is another small canal by which the chorda tympani nerve issues. 

 The outer portion of the Glaserian fissure is entirely closed. 



The f undus of the internal auditory meatus may be ** 



most conveniently studied in an infantile bone, where the 

 canal is short and the apertures relatively wide. A hori- 

 zontal ridge, named crista falcifwmis, runs from the anterior CR ' S FALC " r- ' 

 wall of the meatus across the lamina cribrosa, so as to sepa- FOR - CENT -' 

 rate a small superior from a large inferior fossa. At the 

 bottom of the superior fossa is a collection of minute aper- TBACT - SPIR - FORAM: 

 tures giving passage to the filaments of the superior division of pj^ 43 __ SEMI- DIAGRAMMATIC 

 the auditory nerve, and constituting the area cribrosa superior ; VIEW OF THE FUNDUS OF THE 



while the orifice of the aqueduct of Fallopius is placed on the EIGHT INTERNAL AUDITORY 

 anterior wall of the fossa. In the inferior fossa are seen MEATUS OP AN INFANT. 

 1, the area cribrosa media, below the hinder part of the crest, (Gr. D. T.) j 



for the nerve to the saccule ; 2, the foramen singulars, at 



the lower and posterior part of the fossa, transmitting the nerve of the posterior semicircular 

 canal ; and 3, the tract its sj) trails foraminulentus, for the cochlear division of the auditory 

 nerve, a series of minute holes beginning below the area cribrosa media, forming one turn 

 and a half in a depression corresponding to the base of the cochlea, and ending at the foramen 

 ccntrale cochlea;, the orifice of the central canal of the modiolus. 



From the fore part of the superior border of the petrous portion, where there is often a 

 small projection overhanging the upper end of the groove for the inferior petrosal sinus 

 (fig. 69), a fibrous band, the pctro-sphenoidal ligament, extends to the lateral margin of the 

 dorsum sellas of the sphenoid bone. This completes a foramen through which the inferior 

 petrosal sinus and the sixth nerve pass. In rare cases the ligament is ossified. 



The description of the Small Bones of the Ear, with the Tympanum and Internal Ear, will 

 be found in the chapter on the Organs of the Senses in Vol. III. 



THE SPHENOID BONE. 



The sphenoid 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 nasal fossa?. 

 It is of very irregular shape, and consists of a central part or body, a pair of lateral 

 expansions called the great icings, a pair of smaller horizontal processes above, called 

 the small ivings, and a pair which project downwards, the pteryyoid 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, palate, frontal, and 

 malars, laterally with the squamous portions of the temporals, the parietals, and 

 frontal, and inferiorly with the vomer and palate bones ; sometimes it touches also 

 the superior maxilla. 



The body is hollowed out into two large cavities, the sphenoidal sinuses, 

 separated by a thin median lamina, the sphenoidal septum, and opening anteriorly 

 into the nasal fossas by two rounded apertures. The superior surface presents in 

 the middle a deep pit, the pituitary fossa or sella turcica, which lodges the 

 pituitary body. In front of the fossa is an elevated portion of bone on a level 

 with the optic foramina, the olivary eminence, on which the optic commissure rests 

 in the slight optic groove ; and in front of this is a surface on a somewhat higher level, 

 continuous with the superior surfaces of the smaU wings, and having a slight pro- 



