10 



SUPERFICIAL ANATOMY OF THE HEAD AND NECK. 



The cerebellum, occupying the inferior occipital fossa?, is in contact with the 

 cranial wall up to the lower margin of the transverse part of the lateral sinus. This 

 vessel may occasionally have a lower position than that given on p. 4, and it is 

 advisable, therefore, in operations upon the cerebellum, that the opening in the bone 

 should be kept at least 1 cm. (half an inch) below the level of a line drawn from the 

 external occipital protuberance to the centre of the external auditory meatus, while 

 at the same time it should not extend farther forwards than a vertical line 35 mm. 

 (one inch and a half) behind the latter spot. In this way both the lateral sinus and 

 the occipital artery will be avoided (fig. 9). 



Mastoid antrum. The air-cells, which in the adult usually occupy the interior 

 of the mastoid portion of the temporal bone, open into a small chamber termed the 

 mastoid antrum. This is continuous anteriorly with the highest part of the 



^g sem&tsEular canal 



cctnal offadaL nenie j 



canal of ten&or tytnfianL\ j / 



tube. : .. 



GliuxSiaa, Assure. \VJ& 



tympa., 



A 



\ La&raL sulcus 



Glasenan, fissuss 



e.xt. uwd,. meatus 

 B 



Fig. 6. LKET TEMPORAL BONE, DIVIDED BY A VERTICAL SECTION PASSING THROUGH THE TYMPANUM 

 AND MASTOID ANTRUM : A, INNER PORTION ; B, OUTER PORTION. Natural size. (From a photo- 

 graph by G. W. B. Waters.) (G. D. T.) 



The section is directed somewhat obliquely from before, backwards and outwards. The tympanic 

 cavity and the antrum are coloured blue, and the division between the epityrapanic recess and the 

 antrum is indicated by a dotted line ; c. c. carotid canal. 



tympanic cavity or epitympanic recess (attic of the tympanum), and thus, through 

 the Eustachian tube, the mastoid cells are put into communication with the external 

 air. In form the mastoid antrum may be compared to the bulb of a retort, 

 which is somewhat compressed in the transverse direction, and the truncated neck 

 of which corresponds to the opening into the epifcympanic recess (aditus ad 

 antrum). 



The dimensions of the antrum are subject to considerable variation, but in most 

 cases it measures between 10 and 15 mm. longitudinally, about 10 mm. vertically, 

 and from 4 to 6 mm. transversely. Its depth from the surface, i.e., the thickness 

 of its outer wall, varies from 7 to 14 mm. The bone here is commonly very hard 

 and dense, but in the deeper part it is often more spongy, being excavated by cells 

 in communication with the cavity. The entrance to the antrum from the epi- 

 tympanic recess is rather triangular in form, with the base upwards and the lower 

 angle broadly rounded off : its longest diameter is about 4 mm. both vertically and 

 transversely. The lower margin of the opening is on a level with the upper wall of 



