858 THE ORGANS OF SPECIAL SENSE. 



spine, which is constant in some mammalia. It is supplied by the tympanic 

 branch of the facial nerve. 



Actions. The Tensor tympani draws the membrana tympani inward and thus 

 heightens its tension. The Stapedius draws the head of the stapes backward, and 

 thus causes the base of the bone to rotate on a vertical axis drawn through its own 

 centre : in doing this the back part of the base would be pressed inward toward 

 the vestibule, while the fore part would be draAvn from it. It probably compresses 

 the contents of the vestibule. 



The mucous membrane of the tympanum is continuous with the mucous mem- 

 brane of the pharynx through the Eustachian tube. It invests the ossicula, and 

 the muscles and nerves contained in the tympanic cavity ; forms the internal 

 layer of the membrana tympani, and the outer layer of the membrana tympani 

 secundaria, and is reflected into the mastoid antrum and cells, which it lines 

 throughout. It forms several vascular folds, which extend from the walls of the 

 tympanum to the ossicles ; of these, one descends from the roof of the tympanum 

 to the head of the malleus and upper margin of the body of the incus, a second 

 invests the Stapedius muscle : other folds invest the chorda tympani nerve and the 

 Tensor tympani muscle. These folds separate off pouch-like cavities, and give 

 the interior of the tympanum a somewhat honey-comb appearance. One of these 

 pouches is well marked viz., the pouch of Prussak, which lies between the neck 

 of the malleus and the membrana flaccida. In the tympanum this membrane is 

 pale, thin, slightly vascular, and covered for the most part with columnar ciliated 

 epithelium, but that covering the pyramid, ossicula, and membrana tympani pos- 

 sesses a flattened non-ciliated epithelium. In the antrum and mastoid cells its 

 epithelium is also non-ciliated. In the osseous portion of the Eustachian tube 

 the membrane is thin ; but in the cartilaginous portion it is very thick, highly 

 vascular, covered with ciliated epithelium, and provided with numerous mucous 

 glands. 



The arteries supplying the tympanum are six in number. Two of them are 

 larger than the rest viz., the tympanic branch of the internal maxillary, which 

 supplies the membrana tympani ; and the stylo-mastoid branch of the posterior 

 auricular, which supplies the back part of the tympanum and mastoid cells. 

 The smaller branches are the petrosal branch of the middle meningeal, which 

 enters through the hiatus Fallopii ; a branch from the ascending pharyngeal and 

 another from the Vidian, which accompany the Eustachian tube ; and the tympanic 

 branch from the internal carotid, given off in the carotid canal and perforating 

 the thin anterior wall of the tympanum. 



The veins of the tympanum terminate in the pterygoid plexus and in the 

 superior petrosal sinus. 



The nerves of the tympanum constitute the tympanic plexus, which ramifies 

 upon the surface of the promontory. The plexus is formed by (1) the tympanic 

 branch of the glosso-pharyngeal ; (2) the small deep petrosal nerve ; (3) the 

 small superficial petrosal nerve ; and (4) a branch which joins the great super- 

 ficial petrosal. 



The tympanic branch of the glosso-pharyngeal (Jacobson's nerve) enters the 

 tympanum by an aperture in its floor close to the inner wall and divides into 

 branches, which ramify on the promontory and enter into the formation of the 

 plexus. The small deep petrosal nerve from the carotid plexus of the sympathetic 

 passes through the wall of the carotid canal, and joins the branches of Jacobsou's 

 nerve. The branch to the great superficial petrosal passes through an opening on 

 the inner wall of the tympanum in front of the fenestra ovalis. The small super- 

 ficial petrosal nerve, derived from the otic ganglion, passes through a foramen in 

 the middle fossa of the base of the skull (sometimes the foramen ovale), passes 

 backward and enters the petrous bone through a small aperture, situated external 

 to the hiatus Fallopii on the anterior surface of this bone ; it then courses downward 

 through the bone, and, passing by the gangliform enlargement of the facial nerve, 

 receives a connecting filament from it and enters the tympanic cavity, where 



