INTERNAL EAR. 637 



of the middle meningeal, and branches from the ascending pharyngeal and internal 

 carotid. 



The Veins of the tympanum terminate in the middle meningeal and pharyngeal 

 veins, and, through these, in the internal jugular. 



The Nerves of the tympanum may be divided into : 1. Those supplying the mus- 

 cles. 2. Those distributed to the lining membrane. 3. Branches communicating 

 with other nerves. 



Nerves to muscles. The Tensor tympani is supplied by a branch from the otic 

 ganglion ; the Laxator tympani and the Stapedius, by a filament from the facial 

 (Sommering). 



The nerves distributed to the lining membrane are derived from the tympanic 

 plexus. 



Communications between the following nerves take place in the tympanum ; the 

 tympanic branch, from the petrous ganglion of the glosso-pharyngeal ; a filament 

 from the carotid plexus ; a branch which joins the great superficial petrosal nerve 

 from the Vidian ; and a branch to the otic ganglion (small superficial petrosal 

 nerve). 



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

 panum by an aperture in its floor, close to the inner wall, and ascends on to the 

 promontory. It distributes filaments to the lining membrane of the tympanum, 

 and divides into three branches, which are contained in grooves on the promontory, 

 and serve to connect this with other nerves. One branch runs in a groove, for- 

 wards and downwards, to an aperture situated at the junction of the anterior and 

 inner walls, just above the floor, and enters the carotid canal, to communicate 

 with the carotid plexus of the sympathetic. The second branch is contained in 

 a groove which runs vertically upwards to an aperture on the inner wall of the 

 tympanum, just beneath the anterior pyramid, and in front of the fenestra ovalis. 

 The canal leading from this opens into the hiatus Fallopii, where the nerve 

 contained in it joins the great petrosal nerve. The third branch ascends towards 

 the anterior surface of the petrous bone ; it then passes through a small aperture 

 in the sphenoid and temporal bones to the exterior of the skull, and joins the otic 

 ganglion. As this nerve passes by the gangliform enlargement of the facial, it 

 has a connecting filament with it. 



The chorda tympani quits the facial near the stylo-mastoid foramen, enters 

 the tympanum at the base of the pyramid, and arches forwards across its cavity, 

 between the handle of the malleus and long process of the incus, to an opening 

 internal to the Glaserian fissure. It is invested by a reflection of the lining mem- 

 brane of the tympanum. 



INTERNAL EAR OR LABYRINTH. 



The internal ear is the essential part of the acoustic organ, receiving the ulti- 

 mate distribution of the auditory nerve. It is called the labyrinth, from the com- 

 plexity of its communications, and consists of three parts, the vestibule, semi- 

 circular canals, and cochlea, It consists of a series of cavities, channelled out of 

 the substance of the petrous bone, communicating externally with the cavity of 

 the tympanum, through the fenestra ovalis and fenestra rotunda ; and internally, 

 with the meatus auditorius internus, which contains the auditory nerve. Within the 

 osseous labyrinth is contained the membranous labyrinth, upon which the ramifi- 

 cations of the auditory nerve are distributed. 



The Vestibule is the common central cavity of communication between the parts 

 of the internal ear. It is situated on the inner side of the tympanum, behind the 

 cochlea, and in front of the semicircular canals. It is somewhat ovoidal in shape 

 from before backwards, flattened from side to side, and measures about one-fifth of 

 an inch from before backwards, as well as from above downwards, being narrower 

 from without inwards. On its outer or tympanic wall is the fenestra ovalis, 

 closed, in the recent state, by the base of the stapes, and its annular ligament. 



