THE EAR. 423 



situated in the tympanic bulla. On removing the medial wall 

 of the bulla, a bony plate is seen separating its cavity into two 

 chambers. This plate of bone extends in from the lateral wall 

 of the bulla, forming a complete partition except at the dorso- 

 caudal part, where a notch in the bone forms a free communi- 

 cation between the two chambers. The lateral chamber, or 

 tympanum proper, contains the bones of the ear and is limited 

 externally by the tympanic membrane. At its cranial end is 

 seen the opening of the tuba auditiva, or Eustachian tube. 



The tuba auditiva, or Eustachian tube, is a cartilaginous tube 

 about one and a half to two centimeters long, passing from the 

 nasopharynx to the middle ear and placing these two cavities 

 in communication. The pharyngeal opening is at about the 

 middle of the length of the nasopharynx, near the dorsal border 

 of the lateral wall ; it is a narrow slit two or three millimeters 

 long. Thence the tube passes caudolaterad on the ventral 

 surface of the sphenoid bone, and enters the tympanic cavity 

 through the opening just laterad of the styliform process of the 

 tympanic bulla. The tube has a thick, cartilaginous medial 

 wall, while the lateral wall is thin and formed 

 of connective tissue. The lumen of the tube 

 is a curved slit in cross-section. 



Bones of the Ear (ossicula auditus). There 

 are three small bones in the middle ear, the 

 malleus (hammer) (Fig. 171, 1), incus (anvil) 

 (Fig. 171, 2), and stapes (stirrup) (Fig. 172). FIG. 



The malleus (Fig. 171, i) consists of a ^s AND 



v UNITED IN THE 



slender handle (a) with a small neck (&} at- NATURAL Posi- 



tached at an angle to the caudal end of the TION ' 



1 , r < , *> malleus; 2, in- 



handle. At the end of the neck is a rounded C us. a, handle of 



head (c] which articulates with the incus (2), mal ' eus j i>, neck; c, 



' head; a, process for 

 and together with one part of the incus lies in attachment of ten- 



a small fossa dorsad of the fenestra vestibuli. ^ a f j. ^ w"^ r 

 From the medial aspect of the neck extends a like plate and pro- 

 short process of bone (d] to which is attached tnTf wo processes of 

 the tendon of the tensor tympani muscle. One the incus - 

 side of the neck is produced into a thin wing-like plate, bear- 

 ing on its edge a pointed process (e). 



