530 ANATOMICAL TECRNOLOGT. 



thing like the shell of a snail. The large end of the canal is visible 

 through the membrane covering the Fenestra rotunda, and, since it 

 faces into the tympanic cavity, is called Scala tympani. The other 

 canal opens into the vestibule and is called the Scala vestibuli. 



To demonstrate the parts of the cochlea, rest the head on the 

 occiput, and with a watch-spring saw make a section across the 

 cochlear eminence so that the Fenestra vestibuli is divided in half. 

 If a watch-spring saw is not at hand, one may employ the nippers. 

 Remove the fragments made by the saw or the nippers by blowing 

 with the blowpipe. This will expose the vestibule, a cross section 

 of- the cochlea and the opening of the scala vestibuli. The appear- 

 ance shown in Fig. 127 will be seen, except that the membranous 

 part of the septum may be torn. The direction of the cochlea 

 beyond the vestibule is nearly in a line, connecting the centers of 

 the foramen jugulare et ovale (Fig 57), and if the ventral wall of 

 the cochlear eminence be removed along such a line, the cochlea 

 will be exposed and a clear view obtained both of the Lamina spi- 

 ralis and the Modiolus or center piece around which the coils are 

 made. 



To remove the ventral wall of the cochlear eminence, press a 

 blunt-pointed scalpel or arthrotome against the wall of the scala 

 tympani and pry carefully. Usually it will come off without the 

 least diflBculty. The exposed cochlea will look like a cork- screw. 

 It should be remembered that in addition to the lamina spiralis 

 which forms a partition between the two scalse, there will appear a 

 complete wall of bone separating the different whorls. 



In the center of the modiolus is a cavity or canal, and the lamina 

 spiralis is perforated by many small holes, giving the appearance 

 of a sieve, and under the tripod it is seen that through these the 

 branches of the auditory nerve pass, to be distributed to the sentient 

 part of the cochlea. 



§ 1441. Canales semicirculares (Fig. 127). — There are three of 

 these, each forming about two thirds of a circle, in the periotic bone. 

 They are related somewhat as are related the three dimensions of a 

 cube, and open into the vestibule in pairs. From their position, 

 they are named as follows in man, and the terminology has been 

 retained for the cat : external, superior and posterior. The exter- 

 nal one (horizontal) is nearly in a dextro-sinistral plane and sur- 

 rounds a small fossa nearly caudad of the fenestra ovalis. The 

 superior one is in a dorso-ventral (vertical) transverse plane. It is 



