648 



MAMMALIA 



[CH. 



The ear of Mammalia is distinguished by the development of a 

 region termed the cochlea which is not present or at any rate clearly 

 differentiated in the ears of the lower Vertebrates. It is a spirally 

 coiled outgrowth of the sacculus of the ear, which replaces the 

 simple retort-like lagena which is the outgrowth from the sacculus 

 in Birds and Reptiles. From a study of its minute structure it 

 appears that it is not the homologue of the lagena, which is still 

 represented in Mammals by a slight vesicular enlargement of the tip 



FIG. 320. Diagrammatic transverse section of the bony cochlea and its con- 

 tained sense-organ in a Mammal. 



1. Scala vestibuli. 2. Scala media (the real sense-organ). 3. Scala 



tympani. 4. The pillars of Corti. 5. The tunnel of Corti. 



6. Outer auditory cells. 7. Inner auditory cells. 8. Supporting 

 cells. 9. Basilar membrane. 10. Keissner's membrane. 



11. Membrana tectoria. 12. Auditory nerve. 



of the cochlea, but is rather a special development of the region at 

 the base of the lagena connecting it with the sacculus. The special 

 peculiarity of the cochlea is the organ of Corti, a development of 

 the epithelium forming the basal wall of the cochlea. This consists 

 of a double series of articular rods meeting each other above like the 

 beams of a roof. Between the opposite members of the series is a 

 space, the so-called tunnel of Corti. On the outer side of each 

 series are several rows of sensory (auditory) cells each carrying 



