100 



THE EAR. 



circular canals : and at the lower and fore part of the cavity is a larger opening, 

 which communicates with the scala vestibuli of the cochlea. 



The semicircular canals are three tubes, situated above and behind the vesti- 

 bule, into which they open by five apertures, the contiguous ends of two of the 

 canals being joined. They are unequal in length, but each tube is bent so as to 

 form about two-thirds of an ellipse ; and is moreover dilated at one end, the 

 enlargement being known as the ampulla. The canals are compressed laterally, and 

 measure 1mm. to l'5mm. across ; but the ampulla has a diameter of about 2mm. 



The canals differ from one another in direction, in length, and in position with 

 regard to the vestibule. The superior semicircular canal (fig. 108, 3, fig. 109, 5). 

 19 mm. long, is nearly vertical and lies transversely to the bony axis of the petrous 



s.c.e. s.c.s-p. 



amp 



amp 



aq. Fall. 



pyr. 

 sin.tymp. 



pr. - 



amp.c. 

 aq.Fall. 



, o.Eu.t. 



Fig. 107. PORTIONS OF THE PREVIOUS FIGURE ENLARGED. (Testut.) 



A., anterior ; B., posterior segment, amp.e., amp.s. and amp. p., ampullary orifices of external, 

 superior and posterior semicircular canals ; s.c.e., non-ampullary orifice of the external canal ; s.c.s-p., 

 conjoint non-ampullary orifice of the superior and posterior canals ; aq.Fall., aqueductus Fallopii ; f.o., 

 fenestra ovalis ; f.r., fenestra rotunda ; aq.cochl., aqueductus cochleae ; sc.vest., commencement of scala 

 vestibuli : that of the scala tympani is just below it ; lam.sp., spiral lamina ; n. VII., orifice for facial 

 nerve ; n. VIII. s., orifices for superior or vestibular division of auditory nerve ; tr.sp.for., orifices for 

 cochlear nerve ; f.s., foramen singulare ; pyr., pyramid ; pr., promontory ; sin.tymp., sinus tym- 

 pani ; o.Eu.t., orifice of osseous Enstachian tube. 



bone, forming an angle of about 45 with the coronal plane ; it rises higher than any 

 other part of the labyrinth, and its place is indicated by a smooth arched projection 

 on the upper surface of the petrous bone. The ampullary end of this canal is the 

 external and anterior, and opens by a distinct orifice into the upper part of the 

 vestibule (fig. 107, s) ; whilst the opposite extremity joins the non-dilated end of 

 the posterior semicircular canal, and opens by a common aperture with it into the 

 back part of the vestibule (fig. 107, s.c.s-p. ; fig. 109, 3). The posterior semi- 

 circular canal (fig. 108, 5, fig. 109, 6), is also nearly vertical, and lies in a plane 

 which is almost parallel with the superior canal of the other side (Crum Brown). 

 The posterior and superior canals of the same side incline towards one another at 

 their inner ends. The posterior is the longest of the three (22 mm.) : its ampullary 

 end is at the lower and back part of the vestibule (fig. 107, amp.p) ; and the 

 opposite end terminates in the common canal above described (s.c.s-p.}. The 



