CHAP, ix.] NEEVOUS SYSTEM AND ORGANS OF SENSE. 299 



The auditory ossicles are connected with and suspended from the 

 surrounding bones by delicate ligaments. 



One such suspends the malleus from the wall of the tympanum, 

 and the incus is similarly suspended, while the stapes is connected 

 by ligament with the margin of the fenestra ovalis. 



There are two very small muscles * connected with the auditory 

 ossicles the tensor tympani and the stapedius. 



The tensor tympani muscle arises from a pointed process which 

 projects from the free margin of the septum of the bulla as it 

 curves upwards at the posterior wall of the tympanum. This 

 process is sometimes small, sometimes rather long and pointed. 

 The muscle then runs forwards and is inserted into the processus 

 muscularis of the malleus. 



Beyond the origin of the tensor tympani the septum curves 

 forwards and upwards and is lost on the promontory a little beyond 

 the fenestra ovalis. 



The stapedius muscle arises from a more or less deep pit above 

 the fenestra ovalis. It descends almost vertically to be inserted into 

 the neck of the stapes. 



The mucous membrane of the tympanum is continuous, through 

 the Eustachian tube, with that of the pharynx, and like it, is clothed 

 with a ciliated epithelium, except where it lines the tympanic mem- 

 brane. That membrane is, moreover, coated on either side by other 

 membranes, being lined within by the general lining of the tympanic 

 cavity, as it is coated without by the membrane of the external 

 auditory meatus. 



The INTERNAL EAR or special auditory part of the organ of hearing 

 is a membranous structure enclosed in an excavation of the petrous 

 part of the temporal bone called the BONY LABYRINTH. This laby- 

 rinth consists of three parts, a central chamber, the vestibule, con- 

 nected with a part called the cochlea in front, and behind, with 

 another made up of three semicircular canals. 



The VESTIBULE is a somewhat pyramidal cavity, the front part of 

 the inner wall of which abuts against the bottom of the internal 

 auditory meatus and is pierced by numerous small foramina, 

 which admit the filaments of the auditory nerve. At the front of the 

 lower part of the vestibule is the opening which leads to the cochlea, 

 while the posterior end of the vestibule shows the four openings of 

 the three semi- circular canals. The outer wall of the vestibule is 

 perforated by the fenestra ovalis, which, in the dry skull, opens into 

 the tympanum. 



The COCHLEA (Fig. 138, A, c) is conical, and has the general form 

 of a limpet shell, with its base directed towards the internal auditory 

 meatus. The cone consists of a spiral tube which tapers as it 

 recedes from its base, winding round a central column (the modiolus), 

 the tube being incompletely divided by a lamina of bone, the lamina 



* The origins and insertions of these muscles have been carefully verified by 

 Mr. Alban Doran. 



