INTERNAL EAR LABYRINTH. 461 



first turn of the cochlea. It is channelled upon its surface by three small 

 grooves, which lodge the three tympanic branches of Jacobson's nerve. 



The Foramina and processes of the tympanum may be arranged, accord- 

 .ng to their situation, into four groups. 



1. In the External wall is the meatus audit onus, closed by the mem- 

 brana tympani. 



2. In the Inner wall, from above downwards, are the 



Opening for the tensor tympani, 



Ridge of the aqueeductus Fallopii, 



Fenestra ovalis, 



Promontory, 



Grooves for Jacobson's nerve, 



Fenestra rotunda. 



3. In the Posterior wall are the 



Opening of the mastoid cells, 



Pyramid, 



Opening for the stapedius, 



Opening for Jacobson's nerve, 



Apertura chords (entrance). 



4- In the Anterior wall are the 



Eustachian tube, 



Fissura Glaseri, 



Opening for the laxator tympani, 



Apertura chordae (exit). 



The tympanum is lined by a vascular mucous 7nembrane, which invests 

 the ossicula and chorda tympani, and forms the internal layer of the mem- 

 brana tympani. From the tympanum it is reflected into the mastoid cells, 

 which it lines throughout, and it passes through the Eustachian tube to 

 become continuous with the mucous membrane of the pharynx. 



Vessels and Nerves. The Arteries of the tympanum are derived from 

 the internal maxillary, internal carotid, and posterior auricular. 



Its Nerves are, 1. Minute branches from the facial, which are distri- 

 buted to the stapedius muscle. 2. The chorda tympani, which leaves the 

 facial nerve near the stylo-mastoid foramen, and arches upwards to enter 

 the tympanum at the root of the pyramid ; it then passes forwards between 

 the handle of the malleus and long process of the incus, to its proper 

 opening in the fissura Glaseri. 3. The tympanic branches of Jacobson's 

 nerve, which are distributed to the membranes of the fenestra ovalis and 

 fenestra rotunda, and to the Eustachian tube, and form a plexus by com- 

 municating with the carotid plexus, otic ganglion, and Vidian nerve. 4. 

 A filament from the otic ganglion to the tensor tympani muscle. 



INTERNAL EAR. 



The Internal ear is called labyrinth, from the complexity of its commu- 

 nications; it consists of a membranous and an osseous portion. The 

 39* 



