BOUNDARIES OF THE TYMPANUM. 805 



temporal branches of the external carotid. Its nerves are derived Nerves, 

 from the auriculo-temporal branch of the fifth nerve, and enter the 

 auditory passage between the bone and the cartilage. 



MIDDLE EAR. The chief part of the middle ear is the tympanum Middle ear 

 or drum, a cavity containing air, which is interposed between the 

 external auditory canal and the labyrinth or internal ear. The 

 space is traversed by a chain of small bones, with which special 

 muscles and ligaments are connected. It communicates in front 

 with the pharynx by a canal named the Eustachian tube ; and Eustachian 

 behind, it is prolonged into a series of excavations in the mastoid 

 part of the temporal bone the mastoid cells. Small vessels and ce ii s . 

 nerves ramify in the cavity. 



Dissection. The tympanic cavity should be examined in both a Dissection 

 dried and a recent bone. 



On the dry temporal bone, after removing most of the squamous to open it in 

 portion by means of a vertical cut of the saw through the root of bone?, 17 

 the zygoma and the Glaserian fissure, the tympanum will be brought 

 into view by cutting away with the bone-forceps some of the upper 

 surface of the petrous portion, and the anterior part of the meatus 

 auditorius. 



In the recent bone prepare the dissection as above, but without and in * he 

 ,... . , -111 recent bone, 



doing injury to the niembrana tympani, the chorda tympani nerve, 



and the chain of bones with its muscles. 



The TYMPANUM has the form of a very short cylinder, which is T y m P anum: 

 placed obliquely, so that its end-surfaces (the inner and outer walls form an( * 

 of the tympanum) are nearer to the median plane in front than 

 behind. The circumference of the cylinder is somewhat irregular, 

 and interrupted at parts : in it a roof, a floor, and an anterior and 

 a posterior wall are distinguished. The cavity measures about half dimensions - 

 an inch from above down and from before back. Its breadth may 

 be given as one-sixth of an inch ; but it is wider above and behind 

 than at the lower and fore parts ; and it is narrowest in the 

 centre, owing to the projection towards the cavity of the promontory 

 on the inner wall, and of the tympanic membrane externally. 



The inner boundary of the tympanum (fig. 296) is formed by the Inner wall 

 outer wall of the osseous labyrinth, by the parts of which the con- 

 formation of this surface is mainly determined. Occupying the 

 greater part of the inner wall is a rounded eminence called the is marked by 

 promontory (pr) ; this becomes narrow behind, and its surface is pr01 

 marked by two or three minute grooves which lodge the nerves of and grooves; 

 the tympanic plexus. Above and below the narrowed end of the 

 promontory is an aperture : both lead into the labyrinth. 



The upper aperture (fo) is semicircular in shape, with the con- fenestra 

 vexity upwards, and is named fenestra ovalLs : it opens into the ovalis; 

 vestibule, and into it the inner bone (stapes) of the chain is fixed. 

 The lower aperture, fenestra rotunda (/r), is rather triangular in fenestra 

 form, and is situate within a funnel-shaped hollow : in the macerated 

 bone it leads into the cochlea ; but in the recent state it is closed by 

 a thin membrane the secondary membrane of the tympanum. 



Arching above the fenestra ovalis on this wall is a ridge of 



