THE INTERNAL EAR, OR LABYRINTH 1139 



It is somewhat ovoid in shape, flattened from within outward, and measures 

 about 5 mm. (one-fifth of an inch) from before backward, as well as from above 

 downward, and about 3 mm. (one-eighth of an inch) from without inward. On 

 its outer or tympanic wall is the fenestra ovalis (Jenestra vestibuli), closed, in the 

 recent state, by the base of the stapes, and its annular ligament. On its inner wall, 

 at the fore part, is a small circular depression, the spherical recess (recessus sphaeri- 

 cus), in which the saccule is placed. This recess is perforated, at its anterior and 

 inferior part, by about a dozen minute holes ( macula cribrosa media), for the passage 

 of filaments of the auditory nerve to the saccule. Above and behind this depres- 

 sion isanoblique^idge7thT~cTista~^e^HbuII7~flie anterior extremity of which is 

 called the pyramid (pyramis vestibuli). This ridge bifurcates posteriorly to 

 enclose a small depression, the recessus cochlearis, which is perforated by eight 

 small holes for the passage of filaments of the auditory nerve which supply the 

 posterior end of the ductus cochlearis. An oval depression is placed in the 

 roof and inner wall of the vestibule above and behind the cnsta vestibuli. It is 

 called the fovea hemielliptica, elliptical recess, or spherical recess (recessus ellipticus), 

 and receives the utricle. The pyramid and the adjacent elliptical recess are per- 

 forated by numerous minute foramina (macula cribrosa superior]. The openings 

 in the pyramid transmit filaments from the vestibular nerve to the utricle; the 

 openings in the elliptical recess transmit filaments from the vestibular nerve to 

 the ampullae of the superior and lateral semicircular canals. Below and behind 

 the elliptical recess is a groove which deepens into a canal and is called the aquae- 

 ductus vestibuli. This canal passes to the posterior surface of the petrous portion 

 of the temporal bone and opens as a mere crack between the internal auditory 

 meatus and the groove for the lateral sinus. It transmits a small vein, and con- 

 tains a tubular prolongation of the lining membrane of the vestibule, the ductus 

 endolymphaticus, which ends in a cul-de-sac between the layers of the dura mater 

 within the cranial cavity. Behind, the semicircular canals open into the vestibule 

 by jive orifices. In front is an elliptical opening, which communicates with the 

 scala vestibuli of the cochlea by an orifice, apertura scalae vestibuli cochleae. This 

 opening is bounded below by a thin plate of bone (lamina spiralis ossea), which 

 takes origin from the vestibular floor external to the spherical recess and in the 

 cochlea forms the bony portion of the partition betw r een the scala tympani and the 

 scala vestibuli. In the anterior portion of the vestibular floor is a fissure (fissura 

 vestibuli), which passes into the bony part of the canal of the cochlea. The 

 external boundary of this fissure is a small, thin plate of bone (lamina spiralis 

 secundaria). 



The Bony Semicircular Canals (canales semicirculares ossei ) (Fig. 846) are three 

 bony canals situated above and behind the vestibule. They are of unequal 

 length, compressed from side to side, and each describes the greater part of a 

 circle. They measure about 0.8 mm. (one-thirtieth of an inch) in diameter, and 

 each presents a dilatation at one end, called the ampulla ossea, which measures 

 more than twice the diameter of the tube. These canals open into the vestibule 

 by five orifices, one of the apertures being common to two of the canals. 



The superior semicircular canal (canalis semicircidaris superior), 15 to 20 mm. 

 in length, is vertical in direction, and is placed transversely to the long axis of the 

 petrous portion of the temporal bone, on the anterior surface of which its arch 

 forms a round projection. It describes about two-thirds of a circle. Its outer 

 extremity, which is ampullated, communicates by a distinct orifice with the 

 upper part of the vestibule; the opposite end of the canal, which is not dilated, 

 joins with the corresponding part of the posterior canal to form the crus commune, 

 which opens into the upper and inner part of the vestibule. 



The posterior semicircular canal (canalis semicircularis posterior), also vertical 

 in direction, is directed backward, nearly parallel to the posterior surface of the 



