1084 



SURGICAL AND TOPOGRAPHICAL ANATOMY 



ment of the bony external meatus l)elow, and an imaginary line joining the above 

 boundaries (tig. '660). 'Roughly speaking, if the oritice of the external osseous 

 meatus be bisected horizontally, the ui)per half would be on the level of the mastoid 

 antrum. If this segment be again Isisected vertically, its posterior half would again 

 corres]:>ond to the junction of the antrum and middle ear, and immediately behind 

 this lies the suprameatal fossa' (Macewen). When opening the antrum through 

 this triangle, the operator should work forwards, and inwards, so as to avoid 

 the sigmoid sinus (fig. 660); while, to avoid the facial nerve (fig. 660), he should 

 bring the root of the zygoma and the upper part of the bony meatus as close as 

 possible. The level of the base of the lirain will be a few lines al)ove the posterior 

 root of the zvgoma (fig. 659) and al)Out one-quarter of an inch above the roof of 

 the bon}' meatus (Macewen). 



B. The lower or vertical cells of the mastoid are developed later than is 

 the antrum, and vary much in their contents. In only about twenty per cent, do 

 they contain air. 



The veins passim/ from the mastoid cells and tym'panuin fall into three chief groups 

 — (a) those opening into the lateral sinus; (6) those passing through the mastoid 

 foramen into the occipital and scalp veins; (c) those running through the petro- 

 squamosal suture to the dura mater. As all these veins carry connective-tissue 



Fig. 662. — Horizontal Section of Left Temporal Bone, showing the Various 



Parts of the Ear. 



TYMPANIC RING TYMPANUM 



EXTERNAL MEATUS 



FLOOR OF TENSOR TYMPANI CANAL 

 EUSTACHIAN TUBE 

 COCHLEA 



MASTOID AIR CELLS 



CAROTID CANAL 



INTERNAL MEATUS 

 VESTIBULE I AQUEDUCTUS FALLOPII 



POINTER PASSING THROUGH FENESTRA OVALIS 



sheaths, inflammation may reach — (a) the lateral sinus, causing septic phlebitis; 

 (6) the soft parts outside, causing cellulitis, periostitis, etc. ; (c) the dura mater 

 and brain, leading to meningitis and abscess. 



The sphenoidal sinuses are less imj^ortant surgically, but these points should 

 be remembered: — (1) Fracture through them may lead to l)leeding from the nose, 

 which is thus brought into communication with the middle fossa; (2) the com- 

 munication of their mucous membrane with that of the nose may explain the 

 inveteracy of certain cases of ozsena; (8) here and in the frontal sinuses very 

 dense exostoses are sometimes formed. 



THE SCALP. — The importance of the scalp is best seen froui an examination 

 of its layers (fig. 663). These are — (1) skin; (2) subcutaneous fat and 

 fibrous tissue ; (3) the occipito-frontalis and aponeurosis ; (4 ) the sub- 

 aponeurotic layer of connective tissue; (5) the pericranium and subpericranial 

 connective tissue. 



The first three layers are connected and move together. (1) The union of the 

 skin, and its density, are to enable it to meet pressure, and also to ]irevent it 

 rucking into folds. Furthermore, they account for the extreme ])ain of inflamma- 

 tion here, and the diiUcult}' of raising a blister on the scalp. The presence of 



