1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
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larvae of many species of fish. In species of primitive fish the RohonBeard cells exist throughout the life of the animal, but in the later life
of more advanced animals these cells disappear.
In the cyclostomes the spinal cord is flattened. This may result from
the absence of an intramedullary blood supply within the spinal cord,
In the cyclostomes the dendrites of the neural elements extend from the
ventral gray columns into the dorsal white matter. The dendrites of the
spinal neurons tend to run parallel to the long axis of the spinal cord
and join the nerve fibers in the unmyelinated white columns. Commissural cells are found in the spinal cord of cyclostomes. The axons of these
cells decussate ventral to the central canal and then bifurcate to ascend
or descend within the spinal cord. Muller fibers constitute a conspicuous
system of descending motor coordinating nerve fibers that can be traced
throughout the spinal cord of the lamprey. These nerve fibers do not
decussate in the medulla, thus, differing from the well-myelinated, decussated, and collateral bearing Mauthner axons found in the spinal cord
of bony fish.
The spinal cord of the Chondrichthyes is more differentiated than
that of the cyclostomes. In Chondrichthyes there is an increased number
of well-myelinated fiber tracts. In addition, the gray matter is differentiated into a distinct dorsal and ventral horn. Neurons possess extensive
dendritic fields and individual neurons send dendrites from gray into
white matter. Commissural cell axons and other nerve fibers are numerous in the ventral horn. Commissural cell axons decussate in the midline
of the spinal cord in two different commissures which are situated immediately below the central canal.
The spinal cord of the Osteichthyes shows a general resemblance in
organization to that of the Chondrichthyes. In the bony fish there is a
definite division of the gray matter into dorsal and ventral horns. The
dorsal horn is represented as an undivided mass of gray so that the
gray substance of the spinal cord is in the shape of an inverted Y. The
Rohon-Beard cells disappear in the adult forms of actinopterygians.
However, unique to this group of animals one often finds supramedullary
neurons of large size which can be considered to be in the cervical spinal
cord and medulla oblongata. These giant cells can often be seen with
the naked eye and can be as large as 1 mm in diameter. These cells are
linked by unique synaptic configurations. The synapses are electrically
conducting; therefore, thc supramedullary cells are considered to be
electrotonically coupled ( Bennett et al., 1959a-c, 1967a-d; Hagiwara
and Saito, 1959).
In the bony fish, gray matter contains numerous large motor horn
cells, These usually occur in two groups: One group is located in the
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larvae of many species of fish. In species of primitive fish the RohonBeard cells exist throughout the life of the animal, but in the later life
of more advanced animals these cells disappear.
In the cyclostomes the spinal cord is flattened. This may result from
the absence of an intramedullary blood supply within the spinal cord,
In the cyclostomes the dendrites of the neural elements extend from the
ventral gray columns into the dorsal white matter. The dendrites of the
spinal neurons tend to run parallel to the long axis of the spinal cord
and join the nerve fibers in the unmyelinated white columns. Commissural cells are found in the spinal cord of cyclostomes. The axons of these
cells decussate ventral to the central canal and then bifurcate to ascend
or descend within the spinal cord. Muller fibers constitute a conspicuous
system of descending motor coordinating nerve fibers that can be traced
throughout the spinal cord of the lamprey. These nerve fibers do not
decussate in the medulla, thus, differing from the well-myelinated, decussated, and collateral bearing Mauthner axons found in the spinal cord
of bony fish.
The spinal cord of the Chondrichthyes is more differentiated than
that of the cyclostomes. In Chondrichthyes there is an increased number
of well-myelinated fiber tracts. In addition, the gray matter is differentiated into a distinct dorsal and ventral horn. Neurons possess extensive
dendritic fields and individual neurons send dendrites from gray into
white matter. Commissural cell axons and other nerve fibers are numerous in the ventral horn. Commissural cell axons decussate in the midline
of the spinal cord in two different commissures which are situated immediately below the central canal.
The spinal cord of the Osteichthyes shows a general resemblance in
organization to that of the Chondrichthyes. In the bony fish there is a
definite division of the gray matter into dorsal and ventral horns. The
dorsal horn is represented as an undivided mass of gray so that the
gray substance of the spinal cord is in the shape of an inverted Y. The
Rohon-Beard cells disappear in the adult forms of actinopterygians.
However, unique to this group of animals one often finds supramedullary
neurons of large size which can be considered to be in the cervical spinal
cord and medulla oblongata. These giant cells can often be seen with
the naked eye and can be as large as 1 mm in diameter. These cells are
linked by unique synaptic configurations. The synapses are electrically
conducting; therefore, thc supramedullary cells are considered to be
electrotonically coupled ( Bennett et al., 1959a-c, 1967a-d; Hagiwara
and Saito, 1959).
In the bony fish, gray matter contains numerous large motor horn
cells, These usually occur in two groups: One group is located in the