1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
27
The outer segments of the sensory cells appear to have a structure
similar to cones in that there is a laminated outer structure on which
photosensitive pigments are presurncd to be located. The outer segments
vary in shape within each given receptor and also between different
species of fish studied (Breucker and Horstmann, 1965; Oksche and
Kirschstein, 1967; Rudeberg, 1966, 196th). In addition to photosensitivity, the pineal gland may serve an endocrine function in teleosts
( Pflugfelder, 1953, 1954, 1964). Aldehyde fuchsin-positive granules have
been found in the pineal organ of salmon (Hafeez and Ford, 1967) but
these granules have not been found in all animals studied (Holmgren,
1959; Rasquin, 1958; Rudeberg, 1968a).
The habenular, the second portion of the cpithalamus, receives a
massive afferent input from the pineal gland and a moderate input from
the olfactory region and tclencephalon. The cfferent outflow of the
habenular nuclei is mainly to other thalamic and lower centers.
A dorsal and a ventral thalamus, although present in fish, are difficult
to define because of the problem of determining the sulcus medians
which is the traditional boundary between these two divisions of the
thalamus ( Aronson, 1963; Nieuwenhuys and Bodenheimer, 1966). The
difficulty homologizing many of the different thalamic nuclei has been
intensified by the differences in interpretation of nuclear masses by
various authors. For instance, the lateral geniculate nucleus which is
found in the lateral portion of the dorsal thalamus may be small and
dndeveloped, relatively large and considerably devcloped, or contain a
high degree of complexity so that the nucleus is laminated. The complexity of structure is species specific and thus this nucleus of such
variable structure has not always been properly identified. In addition
to the lateral geniculate we find a rotundus complex of nuclei, pretectal
nuclei, which are neurosecretory as well as neural in function, and other
thalamic nuclei. The dorsal and ventral thalamus send connections to
the hypothalamus, midbrain, and other l o ~ r
neural centers. There is
also a plethora of nerve fibers of passage from the telencephalon through
the thalamus to the hypothalamus.
The hypothalamus contains a number of well-differentiated cellular
masses. In fish the hypothalamus appears to be a major center for the
confluence of information coming from the telencephalon. This afferent
input is mainly derived from the medial and lateral forebrain bundles
which also terminate in the preoptic area of the thalamus (Nieuwenhuys, 1967b). Nerve fibers from gustatory regions ascend into the hypothalamus as well as fibers from the ncoustico-lateralis system. Efferent
pathways from the hypothalamus lead to the various parts of the telencephalon and motor centers of medulla, to portions of the dorsal thal-
27
The outer segments of the sensory cells appear to have a structure
similar to cones in that there is a laminated outer structure on which
photosensitive pigments are presurncd to be located. The outer segments
vary in shape within each given receptor and also between different
species of fish studied (Breucker and Horstmann, 1965; Oksche and
Kirschstein, 1967; Rudeberg, 1966, 196th). In addition to photosensitivity, the pineal gland may serve an endocrine function in teleosts
( Pflugfelder, 1953, 1954, 1964). Aldehyde fuchsin-positive granules have
been found in the pineal organ of salmon (Hafeez and Ford, 1967) but
these granules have not been found in all animals studied (Holmgren,
1959; Rasquin, 1958; Rudeberg, 1968a).
The habenular, the second portion of the cpithalamus, receives a
massive afferent input from the pineal gland and a moderate input from
the olfactory region and tclencephalon. The cfferent outflow of the
habenular nuclei is mainly to other thalamic and lower centers.
A dorsal and a ventral thalamus, although present in fish, are difficult
to define because of the problem of determining the sulcus medians
which is the traditional boundary between these two divisions of the
thalamus ( Aronson, 1963; Nieuwenhuys and Bodenheimer, 1966). The
difficulty homologizing many of the different thalamic nuclei has been
intensified by the differences in interpretation of nuclear masses by
various authors. For instance, the lateral geniculate nucleus which is
found in the lateral portion of the dorsal thalamus may be small and
dndeveloped, relatively large and considerably devcloped, or contain a
high degree of complexity so that the nucleus is laminated. The complexity of structure is species specific and thus this nucleus of such
variable structure has not always been properly identified. In addition
to the lateral geniculate we find a rotundus complex of nuclei, pretectal
nuclei, which are neurosecretory as well as neural in function, and other
thalamic nuclei. The dorsal and ventral thalamus send connections to
the hypothalamus, midbrain, and other l o ~ r
neural centers. There is
also a plethora of nerve fibers of passage from the telencephalon through
the thalamus to the hypothalamus.
The hypothalamus contains a number of well-differentiated cellular
masses. In fish the hypothalamus appears to be a major center for the
confluence of information coming from the telencephalon. This afferent
input is mainly derived from the medial and lateral forebrain bundles
which also terminate in the preoptic area of the thalamus (Nieuwenhuys, 1967b). Nerve fibers from gustatory regions ascend into the hypothalamus as well as fibers from the ncoustico-lateralis system. Efferent
pathways from the hypothalamus lead to the various parts of the telencephalon and motor centers of medulla, to portions of the dorsal thal-