610 



THE EAR 



within the external auditory canal. Its outer wall is largely formed 

 by the tympanic or drum membrane, its inner by the osseous wall 

 of the internal ear. 



The contour of the tympanum is very irregular, its cavity being 

 encroached upon by numerous bony indentations which are most 

 pronounced in its internal wall. Externally the tympanic mem- 

 brane is attached to a bony and fibro-cartilaginous ring, the annu- 

 lus tympanicus, which projects somewhat into the tympanic cavity. 

 In front, the orifice of the Eustachian tube is marked by a slight 

 cartilaginous projection near the floor of the cavity. 



Above and behind, the tympanic cavity is prolonged into a deep 

 recess, the epitympanic cavity, in the upper part of whose posterior 



FIG. ddd. FROM A SECTION OK HIE MIDDLE AND INTERNAL EAR OF A GUINEA-PIG. 



Near x is the inner portion of the external auditory meatus in longitudinal section ; it 

 is closed by the delicate drum membrane. At the left of the drum membrane within the 

 tympanum is the incus approaching its articulation with the stapes, one arm and the base 

 of which are shown. The base of the stapes, in the foramen ovalis, is directed toward the 

 vestibule, in which sections of the utricle and saccule are seen. The facial nerve lies at 

 the left of the stapes. Hematein and eosin. Photo, x 9. 



wall are the orifices of the mastoid cells. The upper portion of the 

 cavity contains the rounded heads of the malleus and incus, the two 

 largest of the auditory ossicles. The internal wall of the tympanum 

 presents anteriorly a bulging prominence which is known as the 



