300 



THE CAT. 



[CHAP. ix. 



spiralis, which projects out from the modiolus towards the opposite 

 wall of the tube, except at its apex. It makes three revolutions 

 and one quarter of a revolution round the modiolus, its course 

 being from left to right in the right ear and from right to left in 

 the left ear. 



The two divisions of the tube, incompletely separated by the 

 lamina spiralis, are termed " scales." 



The lower of these is called the scala tympani, and it commences 



at the fenestra rotunda. The 

 other division, called the scala 

 vestibuli, commences at the ves- 

 tibule, with which it freely com- 

 municates. 



The SEMICIRCULAR CANALS 



are bony tubes extending 

 upwards and backwards from 

 the vestibule from the four 

 openings already mentioned 

 as existing in the posterior 

 part of that cavity. 



The tubes describe about 

 two-thirds of a circle each, 

 and may have one end dilated 

 into what is called an ampulla, 

 the cavity being considerably 

 increased where such dilata- 

 tion exists. The three canals 

 receive different names, accord- 

 ing to their positions. 



The superior vertical semi- 

 circular canal (Fig. 138 A, c 3 ) 

 arches upwards and somewhat 

 backwards as well as inwards 

 from the outer side of the 

 skull. It is the presence of 

 this canal which causes a 

 prominence on the upper part 

 of the inner surface of the 



petrous part of the petrous bone, just above the cerebellar fossa. The 

 more anterior and outer end of this arch is the one that dilates into 

 an ampulla and opens by a distinct aperture. Its posterior and 

 inner end joins with the upper end of the arch next to be described, 

 the two opening into the vestibule by a common aperture. 



The posterior vertical semicircular canal (c 2 ) arches backwards 

 and slightly outwards, its upper end starting from the vestibule 

 from an aperture common to it and to the inner end of the superior 

 vertical semicircular canal. Its lower end opens into the vestibule 

 in common with the adjacent end of the external or horizontal semi- 

 circular canal. This last canal arches backwards and outwards and 



Tig. 138. BONY LABYRINTH AND AUDITORY 

 OSSICLES (ENLARGED). 



A. Bony labyrinth of the right ear, extracted 



and reversed in position, its hinder end 

 being placed forwards, and its upper margin 

 downwards. 



c. Cochlea. 



c 1 . External horizontal semi-circular canal. 



c 2 . Posterior vertical semi-circular canal. 



c 3 . Anterior vertical semi-circular canal. 



f 1 . Fenestra rotunda. 



/ 3 . Fenestra ovalis. 



m. Ampullae. 



B. Malleus. 



as. Articular surface of head. 



n. Neck. 



I. Lamina. 



pg. Processus gracilis. 



pm. Proeessus muscularis. 



pb. Processus brevis. 



emn. Manubrium. 



C. Incus and orbiculare. 



o. Cms longum. 



D. Stapes. 



