424 SENSE-ORGANS AND INTEGUMENT. 



The incus (Fig. 171, 2) bears much resemblance to a two- 

 fanged molar tooth. It lies in the fossa with the head of the 

 malleus. Its head or crown is directed craniad and has a 

 concave surface which articulates with the head of the malleus. 

 One of the fangs (_/") extends caudad in the fossa above men- 

 tioned and is held in position by a ligament. The other fang 

 (g) extends ventromediad and articulates with the head of the 

 stapes. This fang terminates in a minute rounded tubercle 

 known as the lenticular process ; this in early stages of devel- 

 opment is a separate bone, the OS lenticulare. 



The stapes (Fig. 172) has the form of a stirrup, and is the 

 last in the chain of bones. The small end is termed the head 

 (a], and the broader part the base (b*). The head 

 of the stapes articulates with one of the processes 

 of the incus. The base of the stapes fits into the 

 fenestra vestibuli of the petrous bone and com- 

 FIG. 172. pletely closes it. The stapes is held in position 

 , by the stapedius muscle, which is attached to a 



, head; b, J 



base; c, process minute process (<:) near the head. 



o f r the att s tapedius ThuS the three bones f the midle ear form a 



muscle. chain which connects the medial surface of the 



tympanic membrane with the fenestra vestibuli. As the 

 fenestra vestibuli forms a portion of the wall of the vestibule, 

 the vibrations of the tympanic membrane are transmitted 

 through the chain of bones to the fluid within the vestibule. 



Muscles of the Middle Ear. 



M. tensor tympani. The tensor tympani muscle is a short 

 conical muscle having origin in a small fossa (Fig. 25, d, p. 

 34) in the petrous bone dorsocraniad of the fenestra vestibuli. 

 Its very short tendon passes laterad and is inserted into a pro- 

 jection (Fig. 171, d] on the neck of the malleus. 



M. stapedius. The stapedius muscle is a short muscle 

 taking origin from a fossa (Fig. 25, /) in the lateral surface of 

 the petrous bone caudad of that for the incus. It is inserted 

 into the head of the stapes. 



The Internal Ear (Fig. 173). 



The internal ear or labyrinth is completely enclosed in the 

 petrous bone. In the dry bone the internal ear communicates 



