THE EXTERNAL EAR. 527 



leading to the membrana tympani. Both are lined with skin, which 

 extends also over the membrana tympani. 



(B) Tympanum s. Auris media (Fig. 127). The middle ear is an 

 elliptical cavity in the bone, lined with mncous membrane and con- 

 taining the bones of the ear and their muscles and the chorda tym- 

 pani nerve. Into it opens the Eustachian canal (Fig. 58, 88), which 

 pnts it into communication with the pharynx. 



(C) Labyrinthus s. Auris entalis (interna), (Fig. 127). The 

 ental (internal) ear or labyrinth is the sentient portion of the ear. 

 It consists of three parts, a common cavity (vestibulum\ from which 

 extend the other two in one direction the canales semicircular es, 

 in the other the cochlea. In the living body the cavity of the inter- 

 nal ear is closed and lined with a thin periosteal membrane, but in 

 the macerated skull the fenestra rotunda and fenestra ovalis put it 

 into communication with the tympanum. 



Dissection. Remove the head of a young cat and wash away 

 the blood. 



1432. Auris ectalis. The external ear. The outer prominent 

 part turns its concave surface latero-cephalad. Note its flexibility 

 and elasticity, also the little pocket on its lateral border (Fig. 87). 

 Spread the edges apart. Note the numerous ridges and winding 

 ways, and that it is only partially covered by conspicuous hairs. 

 At its latero-ectal aspect is the opening into the Meatus auditorius 

 ectaliS) the walls of which are firm and, near its termination, bony. 



Remove the mandible, os hyoides, tongue and larynx, also the 

 skin from one side, including the external ear. Then isolate the 

 facial (7th) nerve as it emerges from the Fm. stylo-mastoideum (Fig. 



57, Fm. stm.). Partially isolate also the Eustachian canal. The 

 opening of the canal will be exposed as shown in Fig. 88, by slitting the 

 soft palate lengthwise and turning the flaps aside. From this point 

 the canal extends cando-laterad to the bony Eustachian tube (Fig. 



58, Cn. Eu.). Remove from around the bulla (Fig. 57) everything 

 except the Eustachian canal and the facial nerve. Cut the carti- 

 laginous part of the meatus where it joins the bony part, but do 

 not allow the instrument to enter the latter. 



1433. Membrana tympani (Fig. 127). Place the head in a 

 good light and look into the bony ectal meatus. A nearly trans- 

 parent membrane will be seen, the Membrana tympani. It sep- 

 arates the ectal ear from the tympanum. Note the white rod (handle 

 of the malleus) extending across its dorsal third. 



