192 



OSTEOLOGY. 



roofed in by the thin tegmen tympani, which separates it from the middle cranial fossa. 



The obliquity of the medial end of the external acoustic meatus, together with the 



groove for the attachment of 

 the tympanic membrane, is 

 well seen, and the thickness of 

 the upper wall of that passage 

 is also noteworthy. The floor 

 of the meatus, formed by the 

 tympanic plate, which separ- 

 ates it from the mandibular 

 fossa, is much thinner, but inj 

 the region of the root of the 

 styloid process there is a mass- 

 ing together of dense bone. 



HORIZONTAL SECTION. 



Figure 181 represents a 

 horizontal section passing 

 through the face a little below 

 the level of the inferior orbital 

 margin, cutting through the 

 root of each pterygoid process 

 posteriorly. The nasal cavities 

 and the maxillary sinuses are 

 thus exposed. The nasal cavity 

 is divided slightly below the 

 inferior edge of the middle 

 concha along the line of the 

 middle meatus. The thin 

 partition, which here separates 

 the nose from the maxillary 

 sinus, is cut through, and the 

 aperture into the sinus laid 

 open. In front of this, the 

 canal for the naso-lacrimal duct 

 is cut across, and its relations 

 to the maxillary sinus in front 

 and to the lateral side, and 

 to the nose medially, are well 

 displayed. The form of the 

 maxillary sinus, as exposed, is 

 triangular, the summit of the 

 triangle being directed later- 

 ally towards the root of the 

 zygomatic process. Its anterior 

 wall, which is here stout, is 

 pierced obliquely by the infra-orbital canal which at this point reaches the facial surface 

 of the maxilla at the infra-orbital foramen. Its posterior wall, thin and convex backwards, 

 is directed towards the infra-temporal fossa laterally, and to the pterygo-palatine fossa 

 medially, where it lies in front of the pterygoid processes. The latter fossa has been cut 

 across and is seen to correspond to the interval between the posterior and superior 

 surface of the maxilla, and the anterior aspect of the root of the pterygoid process. 

 Laterally, it is seen to communicate with the infra-temporal fossa by means of the pterygo- 

 maxillary fissure which is here cut across ; medially, it opens into the nose by the spheno- 

 palatine foramen, which is also divided. On one side the anterior orifice of the pterygoid 

 canal is seen opening on to the posterior wall of the fossa. On the other side, the 

 canal has been laid open, by removing its lower wall, so as to expose its whole length 

 as it leads backward to the anterior edge of the foramen lacerum. In the middle 

 line, the nasal septum, here formed by the vomer and perpendicular part of the ethmoid, 

 is shown in section. A line passing through the inferior orbital fissures cuts the zygo- 

 matic arch where the zygomatic process of the temporal articulates with the zygomatic 

 bone. 



16 



10 11 12 13 14 



FIG. 180. VERTICAL SECTION THROUGH THE SKULL IMMEDIATELY 

 IN FRONT OF THE ROOT OF THE STYLOID PROCESS. 



10. Inferior opening of carotid 



1. Cochlea. 



2. Entrance to the antrum. 



3. Sulcus tympanicus. 



4. Tympanic bone. 



5. Tympano-mastoid fissure. 



6. Part of mandibular fossa. 



7. Tympanic cavity (floor). 



8. Styloid process. 



9. Jugular fossa. 



opening 

 canal. 



11. Jugular foramen. 



12. Canalis hypoglossi. 



13. Occipital condyle. 



14. Foramen magnum. 



15. Basi- occipital. 



16. Squamous part of occipital 



bone. 



