HEARING. 



CHAPTER XXXVIII 



DISSECTION OF THE EAR IN THE SKATE. 

 AUDITORY SENSATIONS. 



Ear of Skate. 1 1. Pare away the cartilage between the eyes of a 

 skate. When the brain is reached continue the paring laterally, and in 

 the cartilage at the side of the hinder part of the brain there will 

 eventually be exposed one of the semicircular canals. When this is 

 reached remove the upper wall as far as possible. In the hollow formed 

 by the cartilage will be seen the fine membranous canal, dilating at 

 one end into an ampulla. On continuing the exposure of the mem- 

 branous canal it will be seen to join a rather large membranous sac, the 

 utricle. Separated by a slight constriction is a smaller sac, the saccule, 

 and at the anterior end of this is a small membranous projection which 

 represents the cochlea. 



2. Continue the dissection further so as to completely expose the 

 three semicircular canals. Note also a tube leading towards the sur- 

 face from the utricle, representing the recessus vestibuli. 



3. Observe that the ampullae are more rigidly adherent to the 

 cartilaginous walls than the length of the membranous canals. Open 

 one such ampulla where comparatively free and note the crista 

 acustica running transversely across the tube for about a third of the 

 circumference. 



AUDITORY SENSATIONS. 



Range of Appreciation of Sound. EXPERIMENT. In a room as free 

 from noise as possible, let the subject sit with eyes closed and one ear 

 plugged with cotton-wool. Let a watch be held in a line joining the 



1 A dog-fish can be used for this dissection. 

 2B 



