842 THE APPARATUS OF THE SENSES. 



middle, of a fibrous (and muscular) character (fibres radiating towards the 

 centre, ' and also circular) ; an external, epidermic ; and an internal, the 

 mucous membrane of the middle ear. It has vessels and nerves in the 

 external and internal layers. (This membrane receives those vibrations of 

 the air which set in movement the chain of bones in the ear, and thus 

 propagates them to the fenestra ovalis and labyrinth.) 



2. The Promontory, Fenestra Ovalis, and Fenestra Rotunda. 



Placed in the upper part of the tympanic wall, the promontory is only a 

 very small eminence separating the fenestra rotunda from the fenestra ovalis. 

 (It is marked by grooves in which lie the branches of the tympanic nerves.) 



The fenestra ovalis (fenestra vestibuli), situated in front of the promontory, 

 is an opening whose form is sufficiently indicated by its name. It is the 

 opening of communication between the tympanum and osseous vestibule, 

 and is closed by the base of the stapes and the lining membrane of both 

 cavities. 



The fenestra rotunda (fenestra cochlece) is separated from tho preceding 

 by the promontory, and, placed behind this small projection, it is closed in 

 the fresh state by a thin membrane (m. tympani secundaria), that forms a 

 kind of diaphragm between the middle ear and the tympanic scala of the 

 cochlea. (The aqueduct of Fallopius is a canal commencing at the internal 

 ear, passing above the fenestraea and promontory, and terminating at the 

 mastoid foramen. It contains the facial nerve, which passes through the 

 tympanic cavity.) 



3. The Mastoid Cells. 



These cells occupy all the circumference of the tympanic cavity, except 

 above. They are small, more or less irregular, and deep spaces, separated 

 by thin partitions radiating around the tympanic circle, and whose free 

 margin is turned towards the centre of the cavity. 



In several animals, and particularly the Carnivora, the mastoid cells 

 form a special compartment in the tympanic case, communicating with the 

 latter by a single opening. (In the Sheep and Goat, the mastoid cells and 

 their bony septa are entirely absent.) 



4. The Bones of the Middle Ear. (Fig. 395.) 



Four articulating bones (ossicula auditus) named the malleus, incus, os 

 orbiculare, and stapes, compose the bony chain of the middle ear ; this chain 

 extends in a broken course from the external to the internal wall of the 

 tympanum. The pieces are movable on each other, and are united by 

 ligaments and moved by muscles. 



1. MALLEUS (hammer"). This is the longest of the bones, and offers a 

 handle and a head. The handle (manubrium) is placed almost vertical, and 

 firmly fixed to the inner face of the membrana tympani. The head, 

 directed upwards, has a diarthrodial facet for articulation with the incus. 

 The neck, or upper part of the handle, shows two small processes for insertion 

 (processes gracilis and brevis), the innermost of which is very developed. 



2. INCUS (anvil). This bone presents a body or middle portion, and 

 two branches. The body is channeled externally by a diarthrodial facet 

 corresponding to that on the malleus. Of the two branches, the superior 

 terminates in a blunt point; while the other, inferior, is united at its 

 extremity to the os orbiculare. 



