60 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 22. A sagittal section through the mastoid process, the external auditory meatus, and the glenoid fossa. 



FIG. 23. The temporal bone with the exposed tympanum, tympanic antrum, and mastoid cells. The relations 

 of the facial nerve, internal jugular vein, and internal carotid artery to the wall of the tympanum may be seen. The 

 different areas have been colored yellow, blue, and red respectively. 



and occasionally even perforated, so that the nerve is covered only by the tympanic mucous 

 membrane. Inflammations of the middle ear may consequently affect this nerve and produce 

 a facial paralysis. It is not surprising that the nerve may also be involved in caries of the petrous 

 portion of the temporal bone. 



2. Caries in this situation has led to fatal hemorrhage from the internal carotid artery. The 

 vessel is enclosed in the carotid canal in the petrous portion of the temporal bone (see page 34) 

 and, in the region colored red in the illustration, is separated from the tympanic cavity by only a 

 thin bony lamella. If this plate is broken, as in a fracture of the base of the skull, the blood 

 poured out into the tympanic cavity may break through the membrana tympani and escape from 

 the ear or it may go through the Eustachian tube and appear anteriorly at the mouth or nose 

 (see Plate 4). 



3. The jugular fossa, situated on the lower surface of the petrous portion of the temporal 

 bone and accommodating the superior bulb or sinus of the internal jugular vein, is frequently 

 so deep that the inner and lower walls of the middle ear are correspondingly thin and transparent 

 or even perforated. This place was quite thick in the specimen from which the illustration was 

 made. It has been colored blue. 



QUESTIONS. 



Why is it that contusions of the face are not followed by circumscribed swellings, as is the case 

 in the scalp ? 



Where may the pulsations of the facial artery be palpated ? 



How would you proceed in order to expose the infraorbital and the inferior dental nerves ? 



Which large accessory nasal cavity is endangered in neurectomy of the infraorbital nerve ? 



How may the place of exit of the facial nerve be exposed at the base of the skull ? 



What is the explanation of painful mastication and dysphagia in diseases of the parotid gland ? 



What nerve and what vessels are endangered by operations upon the parotid gland ? 



Which of the four walls of the orbit is the strongest? How may emphysema of the orbit and ex- 

 ophthalmos be produced by fracture of any of the other three walls ? What cavities are situated above, 

 below, and to the inner side of the orbit, and what is the direction of growth of tumors originating in 

 these cavities? 



How may the lachrymal gland be easily exposed ? 



What is the relation of the frontal sinus to the lachrymal sac? 



Why is it that a complete retraction of the orbital muscles need not be feared after the division 

 of their tendinous insertions in strabismus operations ? 



Through what thin portions of the base of the skull may the base of the brain be most easily in- 

 jured ? 



Through what portion of what bone may tumors of the base of the skull grow into the nasal fossas 

 with comparative ease? 



What is the explanation of the relief afforded in headache by a pronounced nasal hemorrhage ? 



