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fossa incudis. Some observers describe this as a diarthrodial joint. A superior 
ligament, the ligamentum incudis superius, is sometimes present, but consists mainly 
of a fold of mucous membrane. The vestibular surface, and also the circumference 
of the foot of the stapes, are covered by hyaline cartilage, and a similar layer lines 
the opening of the fenestra ovalis; that encircling the base of the stapes is joined 
to that which lines the fenestra. by a dense ring of elastic fibres, named the 
ligamentum annulare baseos stapedis. The posterior fibres of this annular ligament 
are thicker and shorter than the anterior, and thus the anterior end of the foot- 
plate is free to make greater excursions, during the movements of the bone, than 
the posterior. 
Development of the Tympanic Ossicles.—The malleus and incus are developed 
from the upper end of Meckel’s cartilage (see p- 711). The stapes arises from the meso- 
blast in the region of the fenestra ovalis and is developed around a small artery, the 
stapedial artery, which becomes atrophied in man, but persists in many mammals. 
Muscles of the Tympanic Cavity—These are two in number, viz. m. tensor 
tympani and m. stapedius. 
The m. tensor tympani is the larger, and takes origin from the roof of the cartila- 
ginous part of the Eustachian tube and from the adjacent part of the great wing of 
the sphenoid. It also receives some fibres from the bony canal in which it lies, and 
ends in a tendon which bends outwards, nearly at a right angle to the belly of the 
muscle, round the pulley-like, posterior extremity of the processus cochleariformis. 
Passing across the cavity of the tympanum, this tendon is inserted into the 
inner edge and anterior surface of the manubrium mallei, near its upper end. 
When the muscle contracts it draws inwards the handle of the malleus, and 
so renders tense the membrana tympani; it probably also slightly rotates the malleus 
around its long axis. It receives its nerve from the motor division of the fifth 
cranial nerve through the otic ganglion. 
The m, stapedius arises within the pyramid, and from the canal which prolongs 
the hollow of the pyramid downwards. Its tendon emerges from the apex of the 
pyramid and is inserted into the posterior surface of the neck of the stapes. 
On contraction it draws back the head of the stapes, and so tilts the anterior end 
of the footplate outwards towards the tympanic cavity and the posterior end 
inwards towards the labyrinth, thus rendering tense the heamentum annulare—the 
outward movement of the anterior end of the footplate being greater than the 
inward movement of its posterior end. The muscle is supplied by the facial nerve. 
TYMPANIC OSSICLES. 713: 
Movements of the Tympanic Ossicles.—The manubrium mallei follows all the 
movements of the membrana tympani, while the malleus and incus move together around 
an axis extending forwards through the short process of the incus and the anterior 
ligament of the malleus. When the membrana tympani moves inwards it carries with it 
the handle of the malleus, and the incus, moving inwards at the same time, forces the foot 
of the stapes towards the labyrinth. This inward movement is communicated to the fluid 
(perilymph) in the labyrinth and causes an outward bulging of the secondary tympanic 
membrane, closing the fenestra rotunda. ‘These movements are reversed when the 
membrana tympani is relaxed, unless the outward movement of the membrane be excessive. 
In such a condition the incus does not follow the full outward movement of the malleus, 
but merely glides on this bone at the incudo-malleolar joint, and thus the forcible 
dragging of the foot of the stapes out of the fenestra ovalis is prevented. The cog-tooth 
arrangement, already described, on the head of the malleus and body of the incus, causes 
the incudo-malleolar joint to become locked during the inward movements of the handle 
of the malleus, the joint becoming unlocked during its outward movements. 
Mucous Lining of the Tympanic Cavity (tunica mucosa tympanica).—This is 
continuous, through the Eustachian tube, with that of the naso-pharynx; it also 
extends backwards and lines the mastoid antrum and air-cells. Thin, transparent 
and closely united with the subjacent periosteum, it covers the inner aspect of the 
membrana tympani and is reflected over the ossicles and their ligaments. It also 
supplies sheaths for the tendons of the tensor tympani and stapedius muscles, and 
forms the following mucous folds, viz.: (a) one from the roof of the epitympanic 
recess to the head of the malleus and body of the incus; (0) one enveloping the 
chorda tympani nerve and long process of the incus; (¢) two extending from the 
Se * 
— 
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