TEMPORAL BONES. 



147 



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 meatus auditorius internus, whose size varies considerably ; its mar- 

 gins are smooth and rounded ; and it leads into a short canal, about four lines 

 in length, which runs directly outwards, and is closed by a vertical plate, di- 

 vided by a horizontal crest into two unequal portions. The canal transmits the 

 auditory and facial nerves, and auditory artery. 2. Behind the meatus audi- 

 torius, a small slit, almost hidden by a thin plate of bone, leading to a canal, 

 the aquasductus vestibuli, which transmits a small artery and vein, and lodges 

 a process of the dura mater. 3. In the interval 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. 99) is rough and irregular, and forms part 

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



Fiij. 99. Petrous Portion. Inferior Surface. 



^nir,y of car <Xid <..<, 

 Canal f or Jaco^o 



or Arnold', 9lfrvf 

 Ji'ffuJar fossa 



tyoid process- 

 (Stylo- masfmJ fora men 

 Jucfii,l a .r Surface 

 Auricular fi 



STYLO-PMARYMCEBS 



sents eleven points for examination: 1. A rough surface, quadrilateral in form, 

 which serves partly for the attachment of the Levator Palati and Tensor Tym- 

 pani muscles : 2. The large circular aperture of the carotid canal, which ascends 

 at first vertically, and then, making a bend, runs horizontally forwards and 

 inwards; it transmits the internal carotid artery and the carotid plexus: 8. The 

 aquseductus cochleae, a small triangular opening, lying on the inner side of the 

 latter, close to the posterior border of the petrous portion ; it transmits a vein 

 from the cochlea, which joins the internal jugular: 4. Behind these openings 

 a deep depression, the jugular fossa, which varies in depth and size in different 

 skulls ; it lodges the internal jugular vein, and, with a similar depression on 

 the margin of the occipital bone, forms the foramen lacerum posterius: 5. A 

 small foramen for the passage of Jacobson's nerve (the tympanic branch of the 

 glosso-pharyngeal), this foramen is seen in front of the bony ridge dividing 



