70
JERALD J. BERNSTEIN
dorsomedial gray and is efferent to the trunk musculature, and one group
is located in the ventral gray and is efferent to more specialized areas
such as the pectoral fin (Ariens Kappers et al., 1960). As in the sharks
and rays, the spinal cord of bony fish contains many commissural neurons
and nerve fibers. Many nerve fibers are connected to the propriospinal
system and may ascend to the medulla oblongata, cerebellum, or perhaps
the roof of the midbrain (Nieuwenhuys, 1964). The descending pathways in the bony fish consist mainly of vestibulospinal and reticulospinal
fibers. One of the conspicuous elements of the spinal cord of bony fish
is the Iarge Mauthner fibers which give rise to many short collaterals
within the spinal cord. These short collaterals have extensive connections
with motor horn cells in the ventral horn.
These features are found to be rather constant in all fish, including
some of the most highly evolved bony fish, the Crossopterygi and the
Dipnoi. The spinal cord of fish seems to have reached this organization
early within the evolution of the animal. One finds the massiveness of
the gray matter increases and the connections which are made by the
interstitial cells in the spinal cord become more complex as the evolutionary scale is advanced within the fish from the cyclostomes through
the Osteichythes ( Nieuwenhuys, 1964).
B. Caudal Neurosecretory System and Urophysis
In the actinopterygians the urophysis is a definite neurohemal organ
comprised of a high density of blood vessels and the axonal terminations
of the neurons of the caudal neurosecretory system. In the teleost the
urophysis is a definite organ that is situated ventral to the spinal cord
but is extremely variable in shape (Bern and Takasugi, 1962; Fridberg,
1%2a,b; Fridberg and Bern, 1968; Hamana, 1962). The neurohemal
organ is innervated by a nerve tract which may form an infundibulum or
stalk from the neurosecretory cells within the spinal cord. In contrast
to actinopterygians the elasmobranch urophysis is not a distinct organ
(Bern and Hagadorn, 1959; Fridberg, 1959, 1962b; Hamana, 1962).
However, there is an extensive complex of blood vessels ventral to the
spinal cord that receives scattered neural input from the neurosecretory
neurons that form the caudal neurosecretory system.
The caudal neurosecretory system of fish is composed of neurosecrctory neurons (Dahlgren cells) located within the spinal cord ( Fridberg
and Bern, 1968). In the elasniobranchs the caudal neurosccretory system
is extensive with neurosecretory cells found in as much as 22 segments
of the caudal spinal cord. These specialized neurons occur in two size
JERALD J. BERNSTEIN
dorsomedial gray and is efferent to the trunk musculature, and one group
is located in the ventral gray and is efferent to more specialized areas
such as the pectoral fin (Ariens Kappers et al., 1960). As in the sharks
and rays, the spinal cord of bony fish contains many commissural neurons
and nerve fibers. Many nerve fibers are connected to the propriospinal
system and may ascend to the medulla oblongata, cerebellum, or perhaps
the roof of the midbrain (Nieuwenhuys, 1964). The descending pathways in the bony fish consist mainly of vestibulospinal and reticulospinal
fibers. One of the conspicuous elements of the spinal cord of bony fish
is the Iarge Mauthner fibers which give rise to many short collaterals
within the spinal cord. These short collaterals have extensive connections
with motor horn cells in the ventral horn.
These features are found to be rather constant in all fish, including
some of the most highly evolved bony fish, the Crossopterygi and the
Dipnoi. The spinal cord of fish seems to have reached this organization
early within the evolution of the animal. One finds the massiveness of
the gray matter increases and the connections which are made by the
interstitial cells in the spinal cord become more complex as the evolutionary scale is advanced within the fish from the cyclostomes through
the Osteichythes ( Nieuwenhuys, 1964).
B. Caudal Neurosecretory System and Urophysis
In the actinopterygians the urophysis is a definite neurohemal organ
comprised of a high density of blood vessels and the axonal terminations
of the neurons of the caudal neurosecretory system. In the teleost the
urophysis is a definite organ that is situated ventral to the spinal cord
but is extremely variable in shape (Bern and Takasugi, 1962; Fridberg,
1%2a,b; Fridberg and Bern, 1968; Hamana, 1962). The neurohemal
organ is innervated by a nerve tract which may form an infundibulum or
stalk from the neurosecretory cells within the spinal cord. In contrast
to actinopterygians the elasmobranch urophysis is not a distinct organ
(Bern and Hagadorn, 1959; Fridberg, 1959, 1962b; Hamana, 1962).
However, there is an extensive complex of blood vessels ventral to the
spinal cord that receives scattered neural input from the neurosecretory
neurons that form the caudal neurosecretory system.
The caudal neurosecretory system of fish is composed of neurosecrctory neurons (Dahlgren cells) located within the spinal cord ( Fridberg
and Bern, 1968). In the elasniobranchs the caudal neurosccretory system
is extensive with neurosecretory cells found in as much as 22 segments
of the caudal spinal cord. These specialized neurons occur in two size