THE ORGAN OF HEARING. 59 



the extrusion of pieces of bone into the pharynx. The median sagittal section illustrated in 

 Plate 4 shows that tumors proceeding from the anterior surface of the cervical vertebras, from 

 the clivus, or from the body of the sphenoid meet with little resistance in the direction of the 

 pharynx, and may grow through the choanae into the nasal fossas and then extend into the acces- 

 sory nasal cavities or through the cribriform plate into the cranial cavity ; they may also push 

 down the soft palate and grow toward the mouth. 



THE ORGAN OF HEARING. 



The cartilaginous meatus is bounded anteriorly by the parotid gland, and tumors and in- 

 flammatory swellings of the gland may consequently lead to a narrowing of the auditory canal. 

 The anterior wall of the bony meatus is formed by the thin tympanic plate of the temporal bone. 

 This tympanic plate is the partition between the temporo-maxillary articulation and the external 

 auditory meatus (see Fig. 22), through which affections of this joint may extend into the meatus. 

 This bony plate is occasionally fractured and displaced into the meatus when the condyle 

 of the jaw is driven backward by a blow upon the chin. The posterior wall of the bony meatus 

 varies greatly in thickness; it is often nothing more than a thin lamella of bone, so that the 

 cells of the mastoid process are in close proximity (see Fig. 22). If pus is present in these cells 

 and an early external opening is not provided, it may break into the auditory canal. The trans- 

 verse diameter of the middle ear or cavum tympani is very small, so that the external wall (the 

 drum-membrane) is only about two millimeters from the internal wall (the promontory), and as a 

 result of inflammatory processes the drum-membrane may become adherent to the promontory. 



The thin roof of the tympanic cavity (the legmen tympani), situated in the middle cranial 

 fossa, also covers a portion of the so-called attic or epitympanic recess, a large space which connects 

 the tympanum with the mastoid cells. This is the important path by which suppurations in the 

 middle ear so often extend into the cells of the mastoid antrum. The outer wall of the mastoid 

 process must then be opened, since it is rarely so thin that the pus may perforate externally 

 spontaneously and gravitate along the sternoclcidomastoid muscle. The thinness of the tegmen 

 tympani (see Fig. 23) explains the possibility of the destruction of the bone by middle-ear sup- 

 purations and the escape of pus beneath the dura mater or the extension of the inflammation to 

 the meninges by way of the vascular canaliculi. If the suppuration extends to the brain, an 

 abscess is formed in the temporal lobe which rests upon the tegmen tympani (see page 35). Fig. 

 22 distinctly shows another, though practically less important, relation of the mastoid process. 

 It will be observed that the lateral sinus passes in the immediate vicinity of the mastoid process. 

 This explains the extension of suppurative inflammations of the middle ear to the lateral sinus 

 (sinus thrombosis, pyemia) and through the diseased wall of the sinus to the cerebellum, pro- 

 ducing a cerebellar abscess. These important conditions may be best understood by a study of 

 the base of the skull. 



Fig. 23 also shows three other important relations of surrounding structures to the 

 middle ear: 



i. In the region colored yellow, the facial canal, enclosing the facial nerve, projects more or 

 less into the tympanic cavity. In this situation the wall of the canal is thin, often transparent, 



