122 
THE EAR AND HEARING. 
Iii the Cephalopoda (sepia, cuttle, nautilus, &c.), the 
auditory sac is situated in the nervous ganglia at the base 
of the head. The first approach to a labyrinth makes its 
appearance in this class. 
Ascending to the sub-kingdom, Vertebrata, we find the 
organ but little more complicated in the lowest order of fishes. 
In the Cyclostome (round-mouthed sucker fishes, as the 
hag fish), the vestibule, represented by the sac amongst inver¬ 
tebrates, gives off a single annular passage, which may be 
considered as a semicircular canal, containing a few twigs of 
the auditory nerve. Amongst the higher Cyclostomes—as the 
lamprey—two such canals exist, whilst all other fish have 
three holding the same relation to each other as they do 
in man. 
In the higher orders of fish, as just noted, we find the 
important addition of the labyrinth, consisting of three semi¬ 
circular canals communicating with the vestibule, which, with 
the two ampullae, formed by the expansion of the semicircular 
canals, give rise to three cavities at each side of the base of the 
brain. In each of these cavities is suspended an otolith, 
enclosed by the gelatinous endolymph, and enveloped by a 
thin membrane traversed in every direction by minute 
branches of the auditory nerve, forming an exquisite 
suspensory ligament. These otoliths are best studied in 
the skate, herring, sprat, mackerel, or salmon. They 
are calcareous concretions, pulverulent in the cartilaginous 
fishes (as the shark and ray), but hard and stony, and of a 
porcellanous appearance, in the osseous tribes. 
Some rudiment of a tympanic cavity may be found in 
fishes, but there is no trace of a cochlea. In several of the 
species there is a connection between the labyrinth and the 
air bladder, made by a chain of bones, appearing to fore¬ 
shadow the eustacliian tube of the higher vertebrates. In 
the loach the air bladder is exceedingly small, extending 
under only two vertebrie, and is united with the head in this 
way. The skate has rudimentary external ears on the top of 
its head. 
In the Amphibia (proteus, newt, frog, &c.), which are in 
many respects intermediate between the true reptiles and 
fishes, some species have a tympanum, whilst others, like fish, 
are destitute of it. Wherever the tympanum is distinct, 
there is also an eustacliian tube connecting it with the fauces 
(top of throat). 
The true Reptiles (turtles, snakes, lizards, and crocodiles) 
possess constantly a tympanic membrane and cavity con¬ 
taining a chain of bones. A rudimentary cochlea is also 
