CAN ALBS SEMICIRCULABES. 531 



in the ridge forming the caudal boundary of the Fossa appendicu- 

 laris (Fig. 59, Fs. ap.). 



The posterior semicircular canal is in a dorso-ventral (vertical) 

 longitudinal plane, just laterad of the Fm. jgl. (Fig. 56). As stated 

 above, the canals open into the vestibule in pairs. The opening of 

 one end of the posterior and superior canals may be seen by look- 

 ing into the vestibule. The two other openings are situated near 

 the edge of the fenestra ovalis, one laterad and one mesad of the 

 opening just described. The mesal one is the common opening for 

 the posterior and external canals, while the lateral one is for the 

 superior and external canals. These three openings are situated in 

 a line connecting the middle of the Fm. jugulare and of the fenestra 

 ovalis (Fig. 57, Fm. j., Ft. ov.). 



To trace these canals, remove the perioticum from the rest of 

 the skull, and, commencing at the central or common opening of 

 the posterior and superior canals, with the nippers and arthrotome 

 carefully remove the bony walls of the canal. To demonstrate all 

 the canals and their openings, one should take a skidl, cleaned pref- 

 erably by maceration (§ 250), and after separating the perioticum 

 from the rest of the skull, remove the wall from the middle of the 

 length of the various canals (§ 1441), to expose them ; then insert 

 fine bristles in both directions. In this way the three openings of 

 the canals may be found, and the ends of two bristles wiU be found 

 projecting from each opening. 



§ 1442. Aquaeductus cochleae. — This is a canal through the perioticum which trans- 

 mits a vein from the scala tympani. One opening of the canal is near the fenestra rotunda 

 and the other is j ust caudad of the Meatus miditorius internus (entalis). It may be readily 

 demonstrated by inserting a bristle into the scala vestibular opening. It is mentioned so 

 that it may not be mistaken for the opening of a semicircular canal. 



For a sketch of the development of the eye and ear in the pig, see Hunt, D., 1 ; for 

 the external ear passages, 2 ; and for the development of the middle ear, 3. 



Figure 137 was suggested by the diagram of the human ear given in Huxley and Tou- 

 luans. A, 195, Fig. 83. It is meant to represent the three parts of the ear in their relative 

 order. The first division or auris eetalis is removed, except the bony and a small part of 

 the cartilaginous meatus. 



The bones of the ear were placed as nearly as possible in their proper position and 

 outlined with a camera lucida. 



Explanation of Fig. 127. — Aquseductus cochlese (Aq. chl.). — A passage through the 

 petrosum for a vein from the scala tympani (§ 1443). 



Canalis Eustachiana (Cn. Eu.).— The Eustachian canal opening into the cavity of 

 the tympanum or middle ear (§ 1435). 



Canalis semicircularis posterior (P.). — This canal is represented in its whole length 

 and its opening at one end with the external and at the other with the superior canal. 



