THE INTERNAL EAR. 69 



178. Break away the base of the skull internal to the 

 tympanum, so as to open the vestibule, which, if the 

 saw was held in the proper plane while dividing the 

 skull, will not have been injured. The vestibule has 

 three semicircular canals opening into it, and in front 

 is a conical prolongation of its cavity, which represents 

 a rudimentary cochlea. 



a. The semicircular canals are not very easy to 

 trace on a fresh specimen. If they are not 

 recognized, take first a dried macerated skull, 

 and open the vestibule from its base ; clear 

 away the membrane lining it, and note the 

 recesses of its wall : an anterior at the base 

 of the cochlea, a superior, and a posterior ; 

 into these semicircular canals open. The 

 anterior and posterior vertical canals open into 

 the upper recess by their non-ampullary ends ; 

 a fine bristle pushed forward from this recess 

 passes on through the anterior vertical canal 

 and appears again, through its ampuUary en- 

 largement, in the anterior recess. From the 

 posterior recess the posterior vertical and hori- 

 zontal canals proceed ; the former here has its 

 ampulla, and through it a bristle maybe passed 

 to the superior recess. Push another bristle 

 forward from the opening of the horizontal 

 canal ; it will reappear in the anterior recess, 

 where the ampulla of this canal opens. 



b. Having now found the position of the open- 

 ings, trace out the canals on your fresh speci- 

 men. 



179. Returning to the left side of your section of the 



