7. THE LABYRINTH
225
to homologize the anterior transitional area with the sacculus, the vertical
area with the lagena, and the posterior area with the macula neglecta
(de Burlet and Versteegh, 1930) may well have to be revised in the light
of the ultrastructural findings concerning the pattern of hair cell orientation. It is tempting to suggest that the vertical macula, including perhaps
the transitional area between it and the anterior horizontal macula, is the
homolog of the sacculus and the posterior horizontal macula that of the
lagena. It is true, the gnathostome lagena lies usually in a near-vertical
plane at about right angles to that of the sacculus. However, the spatial
dispositions of maculae are subject to great variation in the gnathostomes.
For instance, in some elasmobranch (dogfish) labyrinths a portion of the
usually horizontal utriculus macula lies in a near-vertical plane, whereas
in others (Raja) both the usually vertical sacculus macula and the utriculus macula approach the horizontal. The macula neglecta of the lamprey
may correspond to what de Burlet and Versteegh described as macula
ductus dorsalis.
The macula communis of Myxine is less complex at first sight (Lowenstein and Thornhill, 1971). It lies on the medial aspect of the labyrinth
capsule and lies in a near vertical plane. Its anterior and posterior ends
curve around becoming more horizontal in the process. There is no
anatomically different middle region. However, the hair distribution corresponds closely to that on the three maculae in the lamprey, if one imagines
these to be projected more or less into a single plane.
What does this all signify in terms of function? The similarity between
the horizontal maculae in the lamprey and the anterior and posterior ends
of the macula communis in Myxine encourage the hypothesis that they
may function in much the same way as has been demonstrated in the
elasmobranch Raja clauata. The orientation of the hair cells on these two
maculae can account for excitatory responses to fore-and-aft, lateral, and
diagonal tilting in opposite directions. Whether or not the median vertical
macula of the lamprey participates in gravity reception cannot be ascertained since the localization of the source of impulse responses in electrophysiological experiments on the isolated labyrinth ( Lowenstein,
1970) is not sufficiently accurate. There is, however, circumstantial
evidence for the localization of vibration sensitivity in the vertical macula
as well as in the so-called macula neglecta of the lamprey labyrinth. In
both these sensory epithelia is found an aberrant type of hair cell, the
hair bundle of which consists of an exceptionally tall and stiff kinocilium
accompanied by a bundle of exceptionally short stereocilia. Clear-cut
responses to vibration have been recorded from the eighth nerve of the
lamprey. No such specialized hair cells are found in the macula communis
of Myxine, and there is no evidence that the organ responds to vibration.
225
to homologize the anterior transitional area with the sacculus, the vertical
area with the lagena, and the posterior area with the macula neglecta
(de Burlet and Versteegh, 1930) may well have to be revised in the light
of the ultrastructural findings concerning the pattern of hair cell orientation. It is tempting to suggest that the vertical macula, including perhaps
the transitional area between it and the anterior horizontal macula, is the
homolog of the sacculus and the posterior horizontal macula that of the
lagena. It is true, the gnathostome lagena lies usually in a near-vertical
plane at about right angles to that of the sacculus. However, the spatial
dispositions of maculae are subject to great variation in the gnathostomes.
For instance, in some elasmobranch (dogfish) labyrinths a portion of the
usually horizontal utriculus macula lies in a near-vertical plane, whereas
in others (Raja) both the usually vertical sacculus macula and the utriculus macula approach the horizontal. The macula neglecta of the lamprey
may correspond to what de Burlet and Versteegh described as macula
ductus dorsalis.
The macula communis of Myxine is less complex at first sight (Lowenstein and Thornhill, 1971). It lies on the medial aspect of the labyrinth
capsule and lies in a near vertical plane. Its anterior and posterior ends
curve around becoming more horizontal in the process. There is no
anatomically different middle region. However, the hair distribution corresponds closely to that on the three maculae in the lamprey, if one imagines
these to be projected more or less into a single plane.
What does this all signify in terms of function? The similarity between
the horizontal maculae in the lamprey and the anterior and posterior ends
of the macula communis in Myxine encourage the hypothesis that they
may function in much the same way as has been demonstrated in the
elasmobranch Raja clauata. The orientation of the hair cells on these two
maculae can account for excitatory responses to fore-and-aft, lateral, and
diagonal tilting in opposite directions. Whether or not the median vertical
macula of the lamprey participates in gravity reception cannot be ascertained since the localization of the source of impulse responses in electrophysiological experiments on the isolated labyrinth ( Lowenstein,
1970) is not sufficiently accurate. There is, however, circumstantial
evidence for the localization of vibration sensitivity in the vertical macula
as well as in the so-called macula neglecta of the lamprey labyrinth. In
both these sensory epithelia is found an aberrant type of hair cell, the
hair bundle of which consists of an exceptionally tall and stiff kinocilium
accompanied by a bundle of exceptionally short stereocilia. Clear-cut
responses to vibration have been recorded from the eighth nerve of the
lamprey. No such specialized hair cells are found in the macula communis
of Myxine, and there is no evidence that the organ responds to vibration.