113 6 THE ORGANS OF SPECIAL SENSE 



The muscles of the tympanum (m. ossiculorum auditus) are two: 1 



Tensor tympani. Stapedius. 



The Tensor tympani (m. tensor tympani) (Fig. 839), the larger, is contained in the bony 

 canal above the osseous portion of the Eustachian tube, from which it is separated by the pro- 

 cessus cochleariformis. It arises from the under surface of the petrous bone, from the carti- 

 laginous portion of the Eustachian tube, and from the osseous canal in which it is contained. 

 Passing backward through the canal, it terminates in a slender tendon which enters the tym- 

 panum and makes a sharp bend outward around the extremity of the processus cochleariformis, 

 and is inserted into the handle of the malleus near its root. Its nerve supply is from the motor 

 root of the trigeminal nerve by way of the otic ganglion. 



The Stapedius (m. stapedius) (Fig. 839) arises from the side of a conical cavity hollowed out 

 of the interior of the pyramid; its tendon emerges from the orifice at the apex of the pyramid, 

 and, passing forward, is inserted into the neck of the stapes. Its surface is aponeurotic, its 

 interior fleshy, and its tendon occasionally contains a slender bony spine, which is constant in 

 some mammalia. It is supplied by the tympanic branch of the facial nerve. 



Actions. The Tensor tympani draws the handle of the malleus inward and thus increases 

 the tension of the tympanic membrane. When the Stapedius contracts it draws the head of 

 the stapes backward, and in consequence the anterior end of the foot-plate passes outward 

 toward the tympanum, and the posterior end inward toward the vestibule, and the annular 

 ligament is made tense. It probably compresses the contents of the vestibule. 



Movements of the Ossicles of the Tympanum. The chain of bones is a lever-like arrange- 

 ment, by means of which the vibrations of the membrana tympani are transferred to the mem- 

 brane covering the oval window, and from this to the perilymph in the labyrinth. When the 

 tympanic membrane moves inward, the handle of the malleus moves with it. The movement 

 of the malleus moves the incus, and the movement of the incus drives the foot of the stapes 

 toward the labyrinth. When the handle of the malleus moves inward, the spur on the head 

 becomes locked with the body of the incus. During outward movement it is unlocked. The 

 ordinary outward movement of the drum membrane causes the above-described movements to 

 be reversed. When there is overforcible outward movement the incus does not go outward 

 quite as far as the malleus, but slides at the joint between the malleus and incus. This 

 reluctance of the incus saves the foot of the stapes from being pulled away from the oval 

 window. 



The mucous membrane of the tympanum (tunica mucosa tympanica) is continuous with that 

 of the nasopharynx through the Eustachian tube. It invests the ossicles, and the muscles and 

 nerves contained in the tympanic cavity, forms the internal layer of the membrana tympani, and 

 is reflected into the mastoid antrum and air cells, which it lines throughout. It forms several 

 vascular folds (plica], which extend from the walls of the tympanum to the ossicles, enveloping 

 these as well as the chorda tympani nerve and the Tensor tympani muscle. 



The anterior malleolar fold (plica malleolaris anterior) comes off from the membrana tympani 

 between the anterior edge of the notch of Rivinus and the handle of the malleus, envelops the 

 processus gracilis of the malleus, the anterior ligament of the malleus, and the anterior portion 

 of the chorda tympani nerve, and terminates in a free concave edge (Spalteholz). The pos- 

 terior malleolar fold (plica malleolaris posterior) is the larger of the two. It comes off from the 

 margin of the notch of Rivinus, envelops the external ligament of the malleus, the posterior part 

 of the chorda tympani nerve, is attached to the handle of the malleus, and ends in a free 

 concave margin (Spalteholz). The fold of the incus (plica incudis) takes origin from the roof 

 of the epitympanic recess and passes to the body and short process of the incus; and a similar 

 fold passes from the head of the malleus to the anterior wall of the epitympanic recess. The 

 entire stapes, with its obturator membrane, is enwrapped by the fold of the stapes (plica stapcdis). 

 This fold also ensheaths the tendon of the Stapedius muscle and often reaches to the posterior 

 wall of the cavity of the tympanum. These folds separate off pouch-like cavities, and give the 

 interior of the tympanum a somewhat honeycombed appearance. 



The inferior external pouch of the tympanum or the pouch of Prussak (recessus membranae 

 tympani superior) is between the flaccid portion of the membrana tympani, the external liga- 

 ment of the malleus, and the neck of the malleus. The anterior and posterior maleolar folds 

 with the tympanic membrane form two pouches. These are the anterior and posterior pouches 

 or recesses of Troeltsch (recessus membranae tympani, anterior and posterior). The anterior 

 pouch is blind above and has a slit-like opening below. The posterior pouch is continued into 

 the blind superior pouch of the tympanic membrane. In the tympanum this membrane is pale, 

 thin, slightly vascular, and covered for the most part with columnar ciliated epithelium, but that 



i Two additional muscles have been described as the Mm. laxator tympani major et minor; they correspond 

 to the anterior and lateral ligaments of the malleus. 



