THE TEMPORAL BONES. 177 



forward, and a little downward. It presents for examination a base, an apex, 

 three surfaces, and three borders, and contains, in its interior, the essential parts 

 of the organ of hearing. The base is applied against the internal surface of the 

 squamous and mastoid portions, its upper half being concealed; but its lower 

 half is exposed by the divergence of those two portions of the bone, which brings 

 into view the oval expanded orifice of a canal leading into the tympanum, the 

 wt'Ktus auditorim externus. This canal is situated in front of the mastoid pro- 



. and between the posterior and middle roots of the zygoma; its upper mar- 

 gin is smooth and rounded, but the greater part of its circumference is surrounded 

 by a curved plate of bone, the auditory process, the free margin of which is thick 

 and rough, for the attachment of the cartilage of the external ear. 



The apex of the petrous portion, rough and uneven, is received into the 

 angular interval between the posterior border of the greater wing of the sphenoid 

 and the basilar process of the occipital ; it presents the anterior or internal orifice 

 of the carotid canal, and forms the posterior and external boundary of the foramen 

 lacerum medium. 



The anterior surface of the petrous portion (Fig. 139) forms the posterior part 

 of the middle fossa of the skull. This surface is continuous with the squamous 

 portion, to which it is united by a suture, the temporal or petro-squamous suture, 

 the remains of which are distinct even at a late period of life ; it presents six 

 points for examination : 1, an eminence near the centre, which indicates the sit- 

 uation of the superior semicircular canal ; 2, on the outer side of this eminence 

 a depression indicating the position of the tympanum ; here the layer of bone 

 which separates the tympanum from the cranial cavity is extremely thin, and is 

 known as the tegmen tympani; 3, a shallow groove, sometimes double, leading 

 outward and backward to an oblique opening, the hiatus Fallopii, for the passage 

 of the petrosal branch of the Yidian nerve and the petrosal branch of the middle 

 meningeal artery ; 4. a smaller opening, occasionally seen external to the latter, 

 for the passage of the smaller petrosal nerve ; 5, near the apex of the bone, the 

 termination of the carotid canal, the wall of which in this situation is deficient in 

 front ; 6, above this canal a shallow depression for the reception of the Gasserian 

 ganglion. 



The posterior surface forms the front part of the posterior fossa of the skull, 

 and is continuous with the inner surface of the mastoid portion of the bone. 

 It presents three points for examination : 1. About its centre, a large orifice, the 

 niftttis auditorius internus. whose size varies considerably; its margins are smooth 

 and rounded, and it leads into a short canal, about four lines in length, which 

 runs directly outward and is closed by a vertical plate, the lamina cribrosa, 

 which is divided by a horizontal crest, the crista fal'ifonnis. into two unequal 

 portions; the lower presenting three foramina or sets of foramina; one, just 

 below the posterior part of the crest, consisting of a number of small openings for 

 the nerves to the saccule ; a second, below and posterior to this, for the nerve to 

 the posterior semicircular canal : and a third, in front and below the first, con- 

 sisting of a number of small openings which terminate in the canalis centralis 

 cochleae and transmit the nerve to the cochlea : the upper portion, that above the 

 crista, presents behind a series of small openings for the passage of filaments to 

 the vestibule and superior and external semicircular canal, and, in front, one 

 large opening, the commencement of the aquaeductus Fallopii, for the passage of 

 the facial nerve. 2. Behind the meatus auditorius, a small slit, almost hidden by 

 a thin plate of bone, leading to a canal, the aqnceductus vestibuli, which transmits 

 a small artery and vein and lodges a process of the dura mater. 3. In the inter- 

 val between these two openings, but above them, an angular depression which 

 lodges a process of the dura mater, and transmits a small vein into the cancellous 

 tissue of the bone. 



The inferior or basilar surface (Fig. 141) is rough and irregular, and forms part 

 of the base of the skull. Passing from the apex to the base, this surface presents 

 eleven points for examination : 1, a rough surface, quadrilateral in form, which 



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