MAMMALIA 



301 



to form the optic nerve. It is believed that the retina is acted 

 upon photochemically, undergoing also pigment change and 

 electrical changes. 



The auditory organ, the only remaining element of the 

 acustico-lateralis system of organs, has already been explained 

 so far as the middle and external regions are concerned. The 

 internal labyrinth presents the usual three semicircular canals, 

 a utriculus and a sacculus, but from the last the cochlea has 



Semic. canals 

 Ant. Ext. Post. 



Hall. 



Duct end. 



Utricle 

 Saccule 



vest. 



Duct. coch. 



Scalatymp. 



Duct coch. 



peril. 



FIG. 140. General diagram of the auditory organ, and to the right 

 a magnified view of a section of the cochlea. 



grown out as an important organ in the form of a coiled tube. 

 The whole organ is closed and is kept distended by endolymph 

 from the ductus endolymphaticus. It is embedded in the 

 periotic bone, from which it is separated by perilymph. The 

 cochlea, however, is attached to the wall of the canal in which 

 it is placed throughout its length. It is applied to one face of the 

 canal, and it is drawn to an angle opposite to this to be attached 

 to a shelf which projects from that side of the canal. The 

 effect of this is to divide the bony canal into three canals, the 

 middle one of which is the cochlear duct, the other two con- 

 taining perilymph. In the cochlear region, therefore, the 

 perilymph is separated by the cochlear duct, and the two 



