954 



TEE SENSORY APPARATUSES. 



orliculare, 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 joined by ligaments 

 and moved by muscles. 



1. Malleus {hammer). — This is the longest of the bones — average length, 

 •Oil mm. — and offers a handle and a head, which almost equally divide it. The 

 ha?idle (manubrium) is placed almost vertically, and is 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, 



Fig. 516. 



BONES OF THE MIDDLE EAR OP THE HORSE. 



M, Malleus: 1, handle; 2, head. E, Incus: 1, inferior branch; 2, superior branch; 3, body. L| 

 Os orbiculare. Et, Stapes: 1, summit; 2, 2, branches; 3, base. Me, Muscle of the stapes; o, 

 bony nucleus in the terminal tendon. 



shows two small processes for insertion (processes gracilis and hrevis), the inner- 

 most 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 

 with 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. 



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

 the inferior branch of the incus and stapes. 



4. Stapes (stirrup). — Remarkable 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 resembles that cavity in shape ; 

 and it is maintained in its position by the mucous lining of the tympanum, which 



