7. THE LABYRINTH
209
A. Semicircular Canals
The semicircular canals are curved membranous tubes filled with
labyrinthine fluid. During their development they are folded off the general labyrinthine cavity with which they remain continuous at both ends.
At one end the canals widen into a spherical ampulla which harbors the
sensory apparatus. This consists of a crestlike ridge, the crista ampullaris,
which juts out into the ampullary lumen. The crista carries the sensory
epithelium made up of sensory cells and supporting cells. Ciliary hair
processes project from the top of the sensory cells into the lumen of the
ampulla. They are ensheathed in a jellylike cupula which reaches from
the top of the crista to the opposite wall of the ampulla. The cupula fits
into the ampulla like the blade of a turbine and is therefore deflected by
movements of the endolymph within the canal. Such movements relative
to the wall of the canal take place whenever the canal is subjected to
angular acceleration by movements of the head in space. In the gnathostomes the three semicircular canals are arranged in three planes of space
approximately at right angles to each other. Two lie in a vertical and one
in a horizontal plane when the head occupies its normal position in space.
B. Otolith Organs
Fundamentally all otolith organs are designed on the statocyst principle. There is a sensory epithelium consisting of sensory cells with an
apical system of hair processes and of supporting cells. Above this epithelium lies a mass of crystalline matter, usually composed of calcium salts.
This is specifically heavier than the surrounding endolymph, and the
otolithic mass will therefore “seek the lowest possible level. In doing so
it will slide along the sensory epithelium whenever this slants from the
truly horizontal level. Two types of otoliths are known. The first is solid
and of a constant shape characteristic of the species and in fact of the
specific otolith organ within the labyrinth. Thus, for example, in the
minnow, Phoxinus laevis, the lapillus in the utriculus is near spherical,
the sagitta in the sacculus is arrow-shaped with winglike ridges, and the
asteriscus in the lagena is flat and star-shaped with a horseshoelike ridge
on one side ( Wohlfahrt, 1932). They show annual growth rings by means
of which the age of a bony fish can be fairly accurately assessed.
The second type of otolith is made up of a pastelike mass of crystalline
“statoconia” in a ground substance. The over-all size and shape of such
statolithic masses can be fairly constant, but they appear to be able to
209
A. Semicircular Canals
The semicircular canals are curved membranous tubes filled with
labyrinthine fluid. During their development they are folded off the general labyrinthine cavity with which they remain continuous at both ends.
At one end the canals widen into a spherical ampulla which harbors the
sensory apparatus. This consists of a crestlike ridge, the crista ampullaris,
which juts out into the ampullary lumen. The crista carries the sensory
epithelium made up of sensory cells and supporting cells. Ciliary hair
processes project from the top of the sensory cells into the lumen of the
ampulla. They are ensheathed in a jellylike cupula which reaches from
the top of the crista to the opposite wall of the ampulla. The cupula fits
into the ampulla like the blade of a turbine and is therefore deflected by
movements of the endolymph within the canal. Such movements relative
to the wall of the canal take place whenever the canal is subjected to
angular acceleration by movements of the head in space. In the gnathostomes the three semicircular canals are arranged in three planes of space
approximately at right angles to each other. Two lie in a vertical and one
in a horizontal plane when the head occupies its normal position in space.
B. Otolith Organs
Fundamentally all otolith organs are designed on the statocyst principle. There is a sensory epithelium consisting of sensory cells with an
apical system of hair processes and of supporting cells. Above this epithelium lies a mass of crystalline matter, usually composed of calcium salts.
This is specifically heavier than the surrounding endolymph, and the
otolithic mass will therefore “seek the lowest possible level. In doing so
it will slide along the sensory epithelium whenever this slants from the
truly horizontal level. Two types of otoliths are known. The first is solid
and of a constant shape characteristic of the species and in fact of the
specific otolith organ within the labyrinth. Thus, for example, in the
minnow, Phoxinus laevis, the lapillus in the utriculus is near spherical,
the sagitta in the sacculus is arrow-shaped with winglike ridges, and the
asteriscus in the lagena is flat and star-shaped with a horseshoelike ridge
on one side ( Wohlfahrt, 1932). They show annual growth rings by means
of which the age of a bony fish can be fairly accurately assessed.
The second type of otolith is made up of a pastelike mass of crystalline
“statoconia” in a ground substance. The over-all size and shape of such
statolithic masses can be fairly constant, but they appear to be able to