GENERAL ARRANGEMENT OF THE BONY LABYRINTH. 735 



of the Eustachian tube is intermediate between the hyaline and the fibro- 

 cartilage. 



The circnmflexus, or tensor palati muscle, which has already been de- 

 scribed in connection with deglutition, is attached to the anterior margin, 

 or the hook of the cartilage. The attachments of this muscle have been ac- 

 curately described by Rudinger, who called it the dilator of the tube. 



The action of certain of the muscles of deglutition dilates the pharyngeal 

 opening of the Eustachian tube. If the mouth and nostrils be closed and 

 several repeated acts of deglutition be made, air is drawn from the tympanic 

 cavity, and the atmospheric pressure renders the membrane of the tympanum 

 tense, increasing its concavity. By one or two lateral movements of the 

 jaws, the tube is opened, the pressure of air is equalized and the ear returns 

 to its normal condition. The nerves animating the dilator tubae come from 

 the pneumogastric and are derived from the spinal accessory. 



A smooth, mucous membrane forms a continuous lining for the Eusta- 

 chian tube, the cavity of the tympanum and the mastoid cells. In all parts 

 it is closely adherent to the subjacent tissues, and in the cavity of the tym- 

 panum it is very thin. In the cartilaginous portion of the Eustachian tube 

 there are mucous glands, which are most abundant near the pharyngeal ori- 

 fice and gradually diminish in number toward the osseous portion, in which 

 there are no glands. Throughout the tube the surface of the mucous mem- 

 brane is covered with conoidal cells of ciliated epithelium. The mucous 

 membrane of the tympanic cavity is very thin, consisting of little more than 

 epithelium and a layer of connective tissue. It lines the walls of the cavity 

 and the inner surface of the membrana tympani, is prolonged into the mas- 

 toid cells and covers the ossicles and those portions of the muscles and ten- 

 dons which pass through the tympanum. On the floor of the tympanic 

 cavity and on its anterior, inner and posterior walls, the epithelium is of the 

 conoidal, ciliated variety. On the promontory, roof, ossicles and muscles, the 

 cells are of the pavement- variety and not ciliated, the transition from one 

 form to the other being gradual. The entire mucous membrane contains 

 lymphatics, a plexus of nerve-fibres and nerve-cells, with some peculiar cells, 

 the physiology of which is not understood. 



The above is merely a general sketch of the physiological anatomy of the 

 middle ear, and it will not be necessary to treat more fully of the cavity of 

 the tympanum, the mastoid cells or the Eustachian tube, except as regards 

 certain points in their physiology. The minute anatomy of the membrana 

 tympani and the articulations of the ossicles can be more conveniently con- 

 sidered in connection with the physiology of these parts. 



General Arrangement of the Bony Labyrinth. The internal portion of 

 the auditory apparatus is contained in the petrous portion of the temporal 

 bone. It consists of an irregular cavity, called the vestibule, the three semi- 

 circular canals (13, 14, 15, Fig. 263) and the cochlea (16, Fig. 263). The 

 general arrangement of these parts in situ and their relations to the adja- 

 cent structures are shown in Fig. 263. Fig. 266, showing the bony labyrinth 

 isolated, is from a photograph in Riidinger's atlas. 



