68
JERALD J. BERNSTEIN
a propagated near field. Thus, the sensory receptor may not be the
essential factor in far field localization and ostariophysid fish may well
localize propagated sound waves.
In the herring, Clupea harengus, there is a thin duct from the swim
bladder which extends into two air-filled bullae close to each labyrinth.
This anatomical mechanism apparently acts in the propagation of auditory signals in a manner similar to Weberian ossicles. Extracellular recordings have been made from the acoustic medullary centers of the
herring ( Enger, 1967). Electrodes were introduced into the medulla
through the cerebellum. The first responses were recorded 1 5 2 . 0 mm
below the cerebellar surface and were obtained from 1000 , LL ventrad and
700-800 p in the anterior-posterior direction. This area included the
acoustic medullary lobe and the afferent acoustic input. Acoustic stimulation produced increased { excitatory) or decreased (inhibitory) responses
over the spontaneous activity of medullary acoustic neurons. Responses
were obtained from units to pure tone stimulation for sound frequencies
of 30-4000 Hz at moderate sound pressure (35 dB) and up to 8000 Hz
for high sound pressures (50-60 dB). Acoustic neurons were classified
into two groups according to response to tone stimulation: (1) units
responding by excitation over spontaneous rate to frequencies up to
500 Hz, and ( 2 ) units responding by inhibition or excitation over spontaneous rate up to 2000 Hz or higher. Although there were increased
thresholds for higher frequencies, the most sensitive auditory neurons
had thrcsholds of 20-25 dB at their optimal sound frequencies which
ranged from less than 100 to 1200 Hz for different units. Thus, the tentative audiogram for herring is -20 to 25 dB for frequencies of 30-1200
Hz with an increase in threshold to +20 dB for 3000 Hz, +35 dB for
4000 Hz.
VIII. SPINAL CORD
A. Anatomy
The fish spinal cord occupies the entire vertebral canal (Nieuwenhuys, 1964) and in the actinopterygians ends in a specialization, the
caudal neurosecretory system (Fridberg and Bern, 1968). The cord consists of a series of segments from which a dorsal and a ventral root arise.
In all vertebrates except the petromyzonts the two roots join to form
spinal nerves (Ariens Kappers et al., 1960; Bone, 1963). In the spinal
cord there are large dorsal cells (Rohon-Beard cells) present in the
JERALD J. BERNSTEIN
a propagated near field. Thus, the sensory receptor may not be the
essential factor in far field localization and ostariophysid fish may well
localize propagated sound waves.
In the herring, Clupea harengus, there is a thin duct from the swim
bladder which extends into two air-filled bullae close to each labyrinth.
This anatomical mechanism apparently acts in the propagation of auditory signals in a manner similar to Weberian ossicles. Extracellular recordings have been made from the acoustic medullary centers of the
herring ( Enger, 1967). Electrodes were introduced into the medulla
through the cerebellum. The first responses were recorded 1 5 2 . 0 mm
below the cerebellar surface and were obtained from 1000 , LL ventrad and
700-800 p in the anterior-posterior direction. This area included the
acoustic medullary lobe and the afferent acoustic input. Acoustic stimulation produced increased { excitatory) or decreased (inhibitory) responses
over the spontaneous activity of medullary acoustic neurons. Responses
were obtained from units to pure tone stimulation for sound frequencies
of 30-4000 Hz at moderate sound pressure (35 dB) and up to 8000 Hz
for high sound pressures (50-60 dB). Acoustic neurons were classified
into two groups according to response to tone stimulation: (1) units
responding by excitation over spontaneous rate to frequencies up to
500 Hz, and ( 2 ) units responding by inhibition or excitation over spontaneous rate up to 2000 Hz or higher. Although there were increased
thresholds for higher frequencies, the most sensitive auditory neurons
had thrcsholds of 20-25 dB at their optimal sound frequencies which
ranged from less than 100 to 1200 Hz for different units. Thus, the tentative audiogram for herring is -20 to 25 dB for frequencies of 30-1200
Hz with an increase in threshold to +20 dB for 3000 Hz, +35 dB for
4000 Hz.
VIII. SPINAL CORD
A. Anatomy
The fish spinal cord occupies the entire vertebral canal (Nieuwenhuys, 1964) and in the actinopterygians ends in a specialization, the
caudal neurosecretory system (Fridberg and Bern, 1968). The cord consists of a series of segments from which a dorsal and a ventral root arise.
In all vertebrates except the petromyzonts the two roots join to form
spinal nerves (Ariens Kappers et al., 1960; Bone, 1963). In the spinal
cord there are large dorsal cells (Rohon-Beard cells) present in the