212 



ANIMAL BIOLOGY 



Semicircular 

 canals 



Utriculus 



Endolymphatic 

 duct 



structure in the adult but also by transient remnants in the 

 human embryo. (Fig. 146.) 



4. The Ear 



The ears, or organs of hearing and equilibration, arise as paired 

 depressions of the ectoderm of the head, which, in all Vertebrates 

 above the lower Fishes, lose their connection with the exterior and 

 form the so-called inner ear, or laryrinth. This becomes divided 

 into two chief parts, the sacculus and the utriculus, from which 



are developed three semi- 

 circular canals, one in 

 each plane of space. The 

 sacculus is largely devoted 

 to the reception of vibra- 

 tions of the surrounding 

 medium, that is to hearing 

 in the usual sense of the 

 word. Accordingly the sac- 

 culus becomes progressively 

 differentiated as we ascend 

 the Vertebrate scale — a 

 Fig. 147. -Diagram of the left mem- CO mplex derivative in the 



Mammalian ear being the 

 cochlea. (Fig. 147.) 



On the other hand, the 

 utriculus and the semicircu- 

 lar canals provide for sensations of loss of equilibrium, or orientation 

 of the body in space, and show far less change. It is probable that 

 equilibration is the chief function of the entire labyrinth in Fishes, 

 as it is of the so-called auditory organs of many Invertebrates, such 

 as the Crayfish. With the progressive specialization of the laby- 

 rinth, the essential sensory cells, which are in communication with 

 the brain by the eighth, or auditory nerve, become limited to a 

 few definite areas. These sensory cells are provided with auditory 

 hairs which project into the cavity of the labyrinth and so are 

 stimulated by movements of the fluid which fills it. 



The ears of Fishes lie immediately below the skull roof where 

 they are readily accessible to vibrations transmitted by the water. 

 But with the substitution of air for water as the surrounding 

 medium, there arises the necessity of a more delicate method for 



Ampullae 



Sacculus 



Ampulla 



Lagena 



branous labyrinth of a lower Vertebrate, 

 showing the sacculus, utriculus, and the 

 three semicircular canals. The lagena is a 

 derivative of the sacculus which becomes 

 the cochlea in higher Vertebrates. 



