THE MIDDLE EAR OR TYMPANUM. 843 



3. Os Oebiculaee. — This is a little, circular, discoid bone, included 

 between the inferior branch of the incus and stapes. 



4. Stapes (stirrup). — Eemarkable for its shape, which is exactly that 

 of a stirrup, this bone is placed almost horizontally. Its summit (or head) 

 articulates with the os orbiculare ; its middle part is divided into two 

 branches, having between them an aperture that is closed by the tympanic 

 mucous membrane. Its base is received into the fenestra ovalis, and re- 

 sembles that cavity in shape ; it is maintained in its position by the mucous 



Fig. 395. 



BONES OF THE MiDDtE EAR OF THE HORSE. — From an Unpublished Drawing ly Lavocat. 

 M, Malleus ; 1, Handle ; 2, Head. — B, Incus ; 1, Inferior branch ; 2, Superior branch ; 

 3, Body. — L, Os orbiculare ; e^, Stapes ; 1, Summit ; 2, 2, Branches ; 3, Base, — 

 Me, Muscle of the stapes ; 0, Bony nucleus in the terminal tendon. 



lining of the tympanum, which passes over the stapes, after being reflected 

 around the margin of the fenestra ovalis. 



(These bones transmit the vibrations of the membrana tympani to the 

 fluid in the labyrinth.) 



5. Ligaments op the Auditoet Bones. — We need only mention the 

 existence of these here, as they are too small and unimportant to merit a 

 particular description. 



6. Muscles of the Auditory Bones. — Four muscles have been de- 

 scribed : three destined to move the malleus, and one for the stapes. But 

 two of these being extremely small, and their muscular character doubtful 

 to many anatomists, we will only notice the internal muscle of the malleus 

 and that of the stapes. 



a. The internal muscle of the malleus (tensor tympani, musculus intemus 

 mallei). — This is a little elongated fasciculus, lodged in a particular groove 

 in the mastoid portion of the temporal bone, and arises near the superior 



