292 ELEMENTAEY PHYSIOLOGY. [XXVIII. 



d. The sacculus, a membranous bag not so 

 large as the utriculus and separated from it 

 only by a shallow constriction. 



e. The rudiment of a cochlea appearing as a 

 small projection from the anterior end of the 

 sacculus. 



2. Trace out the three 'semicircular' canals, the 

 horizontal canal, the anterior and posterior 

 vertical canals, the two latter uniting at their 

 non-ampullary ends. Observe that the planes 

 passing through these canals are at right angles 

 to one another. A tube may be noticed running 

 upwards from the utriculus, close to the point of 

 entrance of the two vertical canals, to open on 

 the surface of the body: the recessus vestibuli of 

 mammals is homologous with this. 



3. Cut through a membranous semicircular canal 

 and pull up with forceps the part connected with 

 an ampulla ; the canal separates from the carti- 

 lage readily, but the ampulla is more firmly 

 attached at one spot, the crista acustica, 

 where a branch of the auditory nerve enters; 

 cut through the ampulla at either end, and with 

 a sharp scalpel cut through the nerve close to 

 the cartilage ; cut open the ampulla on the side 

 opposite the entrance of the nerve and note the 

 ridge of the crista acustica running transversely 

 a third to a half way across the tube, where the 

 nerve enters it. 



