THE TEMPORAL BONE. 



i8i 



20I. 



Facial canal 



Crista falci 

 form is 



Tractus spi-- ^ ' ^ 

 ralis 



;|^x Area cribross 

 '^^ superior 



Cut wall of in- 

 ternal meatus 



Area cribrosa 

 media 



Foramen singu- 

 lare 



Bottom of right internal auditory meatus. X 5. 



the temporal and the occipital. The interior of the mastoid process contains spaces, 

 the mastoid cells, to be described later. The size and shape of the masto.d process 

 are very variable. The rough upper border of the mastoid subdivision forms an 

 entering angle with the squamosal, into which fits a sharp point from the lower bor- 

 der of the parietal, which rests on 



it above. Behind and below the Fig. 



mastoid joins the occipital bone. 



The petrous subdivision is 

 an elongated pyramid running for- 

 ward and inward, presenting four 

 surfaces (besides the base covered 

 by the mastoid), four borders, and 

 an apex. The surfaces are the supe- 

 rior, posterior, inferior, and anterior. 

 The superior surface slants 

 forward and downward in the floor 

 of the middle cerebral fossa. It has 

 the following features. Above the 

 apex there is a depression ^ for the 

 Gasserian ganglion. Just external 

 to this the bone is excessively thin 



and often deficient, so as to leave the end of the carotid canal uncovered. Behind 

 the middle of the pyramid is an elevation, nearly at right angles to its long axis, 

 caused by the superior semicircular canal. External to this the surface is made of a 

 very thin plate of bone, the tegnien tympa7ii, which, extending outward from the 

 petrous, forrhs the roof of the tympanum and of its continuation, the Eustachian 

 tube. Externally, this plate bends down into the Glaserian fissure, so that its edge 

 may appear between the squamosal and tympanic portions (Fig. 198). At the inner 



border of the tegmen tympani near 

 its front is a groove leading to a little 

 rent in the bone, the hiatus Fallopii,^ 

 through which passes the great su- 

 perficial petrosal nerve. A minute 

 opening, more external, transmits 

 the smaller superficial petrosal nerve. 

 In youth the outer side of the teg- 

 men is bounded by the petro-squa- 

 vious suture. 



The posterior surface forms 

 a part of the posterior cranial fossa. 

 The chief feature is the inter7ial 

 auditory tneatus,^ a nearly round 

 canal with a slight groove leading 

 to it from the front. Its shorter 

 posterior wall is about five milli- 

 metres long. The canal is closed by 

 a plate of bone, the lafnina cribrosa 

 (Fig. 201), which is divided by the 

 falciform, crest into a smaller fossa 

 above and a larger one below. The 

 former has an opening by which the 

 facial nerve enters its canal, the 

 aqueduct of Fallopius. Branches of 

 the auditory nerve pass through minute openings in both fossae. About one centi- 

 metre behind the meatus is a little cleft, the aquceductus vestibuli,*^ entering the bone 

 obHquely from below. Higher and nearer to the meatus is a minute depression, the 

 remnant of \.):\& Jloccular fossa,^ which is large in some animals and in the infant. It 

 receives a fold of the dura. 



The inferior surface of the petrous presents in front a large rough surface for 



' Irapressio tegmenti. " Hiatus canalis facialis. ^ Meatus acusticus internus. * Apertura externa aquaeductus vestibuli. 

 Fossa snbarcuata. 



Fig. 202. 



Petro-squamous suture 



Tegmen tympani 



Epitympanic 

 space 



Facial canal 



Internal audi- 

 tory meatus 



Internal ear 



Aquseductus 

 cochleae 



Tympanic cavity 



Squamous por- 

 tion 



Glaserian 

 fissure 



External audi' 

 tory canal 



1 ympanic rmg 



Styloid process 



Frontal section through temporal bone, showing the cavities 

 oi the outer, middle, and inner ear and the four sides of the 

 petrous. 



