132 PRACTICAL ANATOMY. 



student. Its small size is no argument against careful study of its medical and 

 surgical importance, its contents, and its relations, since it is of easy access for 

 dissection, and resolvable into the form of a geometrical cube for purposes of 

 aiding the memory and facilitating a comprehension of the relative position of 

 those anatomical structures that make the middle ear of such great importance. 

 The student should not undertake a dissection of this part until he has a clear- 

 cut idea of the position, relation, and importance of each wall of the cube. If 

 he begins his dissection knowing that the drum of the ear forms the outer wall 

 of the tympanum proper, that the Eustachian tube and mastoid antrum commu- 

 nicate with the tympanum by the anterior and posterior walls respectively, then 

 he will surely find these structures and think of them in relation to the walls of 

 a cube. To say nothing of the part played by the tympanum in hearing, this 

 region is of interest and importance for the following reasons : 



1. The temporo-sphenoidal lobe of the brain lies on its roof. 



2. The floor of the tympanum lies on the jugular fossa. 



3. The drum of the ear and chorda tympani are on the external wall. 



4. The mastoid antrum and cells are behind it. 



5. The internal carotid artery is in front of it. 



6. The facial nerve skirts two sides of it. 



7. The internal ear is in close relation with it. 



8. The drum of the ear may become ruptured. 



9. The interossicular joints may become ankylosed. 

 10. Pus may form in the tympanum. 



Size and Subdivisions of the Tympanum. The tympanum is about one-half 

 of an inch in height and one-sixth of an inch in width. It is prolonged forward 

 as the Eustachian tube and backward as the mastoid antrum, so its length is 

 difficult to determine : for practical purposes one-half of an inch is near enough. 

 The subdivisions are the attic and tympanic cavity proper. The latter is quite 

 narrow, and has the membrani tympani or drum as its outer wall ; the former is 

 broader, contains most of the bones of hearing, and has a part of the temporal 

 bone as its outer wall. Remember, the tympanum is not horizontal, but its 

 anterior end slopes downward, forward, and inward to the Eustachian tube ; its 

 posterior end slopes upward, backward, and outward to the mastoid antrum. 



Study of the tympanum in which this cavity is compared to a box. (Fig. 91.) 



1. The roof of the tympanum is a thin plate of bone, the tegmen tympani, 

 separating the tympanum from the middle fossa at the base of the skull. It is 

 perforated by foramina that transmit the petrosal branches of the seventh nerve. 



2. The floor separates the tympanum from the jugular fossa, in which fossa 

 are the internal jugular vein, and the ninth, tenth, and eleventh cranial nerves. 

 In this floor is an aperture, through which Jacobson's nerve, the tympanic branch 

 of the glosso-pharyngeal, passes to form the tympanic plexus. 



3. The outer wall of the tympanum is formed by the drum below and the 

 squamosa above, these two structures corresponding to the tympanum proper, 

 and the attic respectively. This outer wall is pierced by the following openings : 

 (i) The ifcr clionlie posterius, by which the chorda tympani nerve, a branch of the 

 seventh cranial nerve, enters the tympanum ; (2) the iter chordtc aiiterins, by 

 which the chorda tympani leaves the tympanum ; (3) the (i/aseriau Jissitre, 

 through which pass the tympatiic branch of the internal maxillary artery, the 

 processus gracilis of the malleus, and the laxator tympani muscle. 



4. The inner isall, you will remember, is called the fourth surface of the 

 'petrosa, in Morris' analysis of this part of the temporal bone. This inner wall 



contains: (i) A ridge of bone covering the seventh nerve in its passage 

 through the tympanum, as heretofore explained ; (2) the fenestra ovalis, leading 

 into the vestibule, and to which is attached the base of the stapes, through 





