PISCES. 



173 



labyrinth originates as an ectodermic pit, and in the adult still 

 retains a connection with the exterior by means of a narrow 

 tube (aqueductus vestibuli) opening on the top of the head. 



The labyrinth is divided into a central region (vestibule) and 

 three curved tubes, the semicircular canals. The vestibule is 

 again divided into an anterior utriculus, and a posterior sacculus 

 with which the aqueductus vestibuli communicates and which 

 is produced into a process, the cochlea. The semicircular canals 



Fig. 48. DIAGRAMS or THE MEMBRANOUS LABYRINTH (from Bell, after 

 Waldeyer). A, Fish ; B, Bird ; C, Mammal. Internal side of left 

 labyrinth. us, Utriculus and sacculus ; w, utriculus ; s, sacculus ; 

 c, cochlea. 



are known from their position as anterior vertical, posterior 

 vertical, and external horizontal. They open at both ends into 

 the utriculus, and each has a small swelling (ampulla) at one 

 end. This is anterior in the first and last named, posterior in 

 the horizontal canal. The ampullae and parts of the vestibule 

 possess patches of auditory hair-cells, with which fibres of the 

 auditory nerve are connected. 



Each of the eyes is flattened externally and rounded internally. 

 Its wall consists of three coats, the most external of which is 

 the firm sclerotic, supported by cartilage and exhibiting externally 

 a transparent oval area, the cornea. The middle coat (choroid) 

 is a pigmented vascular membrane lining the rounded part of 



