. Operative surgery, for students and practitioners . o the mastoid portion proper,pretty close to the groove which lodges the sigmoid sinus, so thatthere may be but a very thin shell of bone separating the mastoidcells from the sinus. Mastoids vary in different people and uponopposite sides in the same person as to the extent to which thesecells are developed. They begin to develop early in life, but theage differs at which they are found fully developed. From five yearson they are fairly well marked, and it is said that at the age offifteen years they are all developed down to the tip of the
. Operative surgery, for students and practitioners . o the mastoid portion proper,pretty close to the groove which lodges the sigmoid sinus, so thatthere may be but a very thin shell of bone separating the mastoidcells from the sinus. Mastoids vary in different people and uponopposite sides in the same person as to the extent to which thesecells are developed. They begin to develop early in life, but theage differs at which they are found fully developed. From five yearson they are fairly well marked, and it is said that at the age offifteen years they are all developed down to the tip of the say that they do not reach complete development until a fewvears later. Occasionally the septa may undergo a process of rare- ANATOMY OF THE EAR. 47 faction, the septa gradually disappearing and the spaces opening intoeach other until they are all combined in one large space representedby the antrum. Instead of as above described, the structure of thebone may be spongy or it may be excessively dense and without spaces,resembling Fig. 19.—Side of Skull. A, position of opening in skull to expose the ante-rior branch of the middle meningeal (Vogts lines); G, position of opening forcerebellar abscess; MA, location of mastoid antrum (directly in front of circleMA Is the spina supra meatum); P, opening to expose the posterior branchof middle meningeal; R, Reids base-line continued backward to externaloccipital protuberance; 8, dotted lines represent course of lateral (sigmoid)sinus; T8, opening in the skull for abscess of the temporo-sphenoidal lobe. The Anatomy of the Ear. — Changes that occur in the firstvisceral cleft result in the formation of the external and middleear. The internal ear, labyrinth, etc., are formed within the sub-stance of the petrous portion of the temporal hone. The externalfossa, or cleft, develops into the external auditory canal and auricle; 48 HEAD AND FACE. the internal fossa, or cleft, which opens into the pharynx, becomesthe Eu
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