. Diseases of the ear; a text-book for practitioners and students of medicine. l auditory meatus and projecting for a varyingdistance below the level of its floor. Its lower extremity,forming the apex of the cone, is covered by the aponeurosis of the sterno-mastoidmuscle. This muscleis attached not only tothe tip of the process,but also for a consider-able distance along itsinternal aspect. Abovethe^ insertion of themuscle upon the inter-nal surface of the mas-toid is a deep furrow,the digastric groove,which lodges the oc-cipitaUartery and furnishes attachment for the posterior headof the diga
. Diseases of the ear; a text-book for practitioners and students of medicine. l auditory meatus and projecting for a varyingdistance below the level of its floor. Its lower extremity,forming the apex of the cone, is covered by the aponeurosis of the sterno-mastoidmuscle. This muscleis attached not only tothe tip of the process,but also for a consider-able distance along itsinternal aspect. Abovethe^ insertion of themuscle upon the inter-nal surface of the mas-toid is a deep furrow,the digastric groove,which lodges the oc-cipitaUartery and furnishes attachment for the posterior headof the digastric muscle. This bony mass may be pneumatic,diploic, or sclerotic in structure. In the pneumatic mastoid(Fig. 116) there are numerous air spaces throughout the entiremass ; these are irregularly distributed, in some cases lyingalmost immediately below the cortex, while in other instancesthey are situated at considerable depth below the outer sur-face of the bone and are specially numerous upon its anterioraspect; this anterior wall of the mastoid forms the posterior (442). Fig. 116.—The pneumatic mastoid. The sectionshows the relative position of the tympanic vaultand mastoid antrum to each other and to theintracranial surface. (Authors specimen.) I VARIATIONS IN STRUCTURE. 443 wall of the external auditory canal, and when the cells arewell developed in this region the earliest evidences of theirinvolvement in an inflammatory process appears here. Onepneumatic space is constant, and that is the antrum. This cav-ity is irregularly pyramidal in shape, communicates with thetympanic vault by a narrow passage, and varies considerablyin size in different individuals. At birth the antrum is theonly space developed, the others being formed not infrequently find a second cell of considerable sizelocated at the very tip of the process; the out- ^C^. -~s~®S^&K^BS^I^ er bony wall of this 1^/* ?*yr^::r ™ ^ space is often very thin—a fact which is of con-siderable cl
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