1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
33
tegmentum is the inferior border of the third ventricle. Six layers havc
been described in the optic tectum by Ariens Kappers et al. (1960), and
10 layers have been described by Ramon y Cajhl (1952). In cross section
from the pial surface to the third ventricle the zones according to Ariens
Kappers et al. (1960) are as follows:
First is a stratum opticum (layer 1) which contains the primary optic
fibers from the medial and lateral optic tract. A second zone (layer 2)
of alternate layers of nerve fibers and neurons receives strong afferent
connections from tracts such as the ascending spinotectal and bulbartectal tracts. This is followed by a central zone (layer 3 ) of gray matter
which is rich in neurons and interconnections between neurons. This
zone appears to be similar in structure to the mammalian cerebral cortex
(with primative cell types) and contains the neurons of origin for the
larger efferent tracts. The efferent tracts are the tecto-cerebellar, dorsal
and ventral tecto-bulbar, tecto-spinal tract, tecto-commissural tract, and
connections to the dorsal thalamus. The next zone (layer 4 ) is a stratum
of central white matter which contains the axons of the efferent tracts to
lower centers. A white central stratum (layer 5 ) follows which is comprised of efferent nerve fibers whose nerve cells of origin were in the
layer of gray matter. Fascicles of nerve fibers are located in this zone
that decussate in the tectal commissure. The ipsilaterally derived nerve
fibers of the tectal commissure tend to terminate in the contralateral torus
longitudinalis ( a continuation of the tectal gray on the superior border
of the tegmentum). The next stratum (layer 6 ) is a periventricular
stratum of gray matter. This layer is made up of neurons whose axons
enter into the central white matter and are thus some of the efferent
outflow from the optic tectum (Ariens Kappers et al., 1960; Aronson,
1963; Ramon y CajAl, 1952). The superior colliculus (tectum) appears
to have the same general structure throughout the vertebrates. The
superior colliculus can be thought to be made up of a superficial incoming layer or layers of primary optic fibers, a large zone of intermediate
gray substance containing neurons rich in interconnections ( resembling
the cortex of higher mammals) and a lower efferent fiber layer whose
outflow ascends and descends to other neural centers.
The gray zones within the optic tectum of the fish are particularly
rich in connections (Ramon y Cajhl, 1952). The neurons of this area
contain stellate cells which are reminiscent of mammals and other cell
types which appear to be unique to the tectum of lower vertebrates.
The superficial grouping of nerve cells appears to be primitive fusiform
cells, There are also horizontal connecting cells within this plexiform
layer interconnecting the various neural elements within the optic tectum. In addition, there are large primitive pyramidal-like cells that
33
tegmentum is the inferior border of the third ventricle. Six layers havc
been described in the optic tectum by Ariens Kappers et al. (1960), and
10 layers have been described by Ramon y Cajhl (1952). In cross section
from the pial surface to the third ventricle the zones according to Ariens
Kappers et al. (1960) are as follows:
First is a stratum opticum (layer 1) which contains the primary optic
fibers from the medial and lateral optic tract. A second zone (layer 2)
of alternate layers of nerve fibers and neurons receives strong afferent
connections from tracts such as the ascending spinotectal and bulbartectal tracts. This is followed by a central zone (layer 3 ) of gray matter
which is rich in neurons and interconnections between neurons. This
zone appears to be similar in structure to the mammalian cerebral cortex
(with primative cell types) and contains the neurons of origin for the
larger efferent tracts. The efferent tracts are the tecto-cerebellar, dorsal
and ventral tecto-bulbar, tecto-spinal tract, tecto-commissural tract, and
connections to the dorsal thalamus. The next zone (layer 4 ) is a stratum
of central white matter which contains the axons of the efferent tracts to
lower centers. A white central stratum (layer 5 ) follows which is comprised of efferent nerve fibers whose nerve cells of origin were in the
layer of gray matter. Fascicles of nerve fibers are located in this zone
that decussate in the tectal commissure. The ipsilaterally derived nerve
fibers of the tectal commissure tend to terminate in the contralateral torus
longitudinalis ( a continuation of the tectal gray on the superior border
of the tegmentum). The next stratum (layer 6 ) is a periventricular
stratum of gray matter. This layer is made up of neurons whose axons
enter into the central white matter and are thus some of the efferent
outflow from the optic tectum (Ariens Kappers et al., 1960; Aronson,
1963; Ramon y CajAl, 1952). The superior colliculus (tectum) appears
to have the same general structure throughout the vertebrates. The
superior colliculus can be thought to be made up of a superficial incoming layer or layers of primary optic fibers, a large zone of intermediate
gray substance containing neurons rich in interconnections ( resembling
the cortex of higher mammals) and a lower efferent fiber layer whose
outflow ascends and descends to other neural centers.
The gray zones within the optic tectum of the fish are particularly
rich in connections (Ramon y Cajhl, 1952). The neurons of this area
contain stellate cells which are reminiscent of mammals and other cell
types which appear to be unique to the tectum of lower vertebrates.
The superficial grouping of nerve cells appears to be primitive fusiform
cells, There are also horizontal connecting cells within this plexiform
layer interconnecting the various neural elements within the optic tectum. In addition, there are large primitive pyramidal-like cells that