Communication between the pituitary gland and the brain takes place along
nerve fibres to the neurohypophysis (hypothalamic-neurohypophysial tract) and
pars intermedia, or through blood vessels to the pars distalis. The hypothalamicneurohypophysial tract (which also sends some branches to the median eminence)
originates in amphibians and fishes in the praeoptic nucleus, situated in the anterior
region of the hypothalamus, while in the higher tetrapods (amniotes) these fibres
arise from two areas, the paraventricular and supraoptic nuclei. Blood vessels connect neurons, which converge on the hypothalamus (in the region of the median
eminence of lungfishes and tetrapods), with the pars distalis, These capillaries may
form distinct portal vessels, as in some fishes and most tetrapods, or a less well
defined vascular plexus like in many teleosts, In the fishes the arrangement of the
vascular link varies considerably in different species.
The precise anatomical disposition of the tissues of the pituitary gland varies
in the different vertebrate classes. This is summarized in the now classic illustration
of the late J.D. GREEN (Fig . 2.3). One of the more interesting phyletic developments of the tetrapod neurohypophysis is the appearance of a more distinct
median eminence, and its expansion posteriorly to form the pars nervosa (neural
lobe). WINGSTRAND (1966) has suggested that this may be correlated with adaptation to a terrestrial way of life. It is rather interesting that such a change can also
be seen in the lungfishes, which are often considered to be closely associated with
the evolution of the tetrapods. FOLLETT (1963) summarized information about the
hormonal content of the vertebrate neurohypophysis and found its concentration
(moles/kg body wt) to be greater in those species with a well developed pars nervosa .
b) Secretion of the Pituitary Gland
cx.) Pars Distalis. Eight hormones with distinct actions have so far been found to
originate in the pars distalis. These (see Table 2.2) include the gonadotrophic
hormones (FSH and LH or ICSH), which influence breeding cycles, by promoting
the maturation of the germ cells in the ovaries and testes, as well as the production
of the steroid sex hormones. These hormones are sometimes indirectly associated
with a change in the animals' osmotic circumstances. Thus fish, like the salmon and
lamprey, migrate from the sea into rivers in order to breed while eels move from
rivers back into the sea for the same reason. Periodic breeding migration of birds,
mammals and reptiles does not result in such dramatic changes in the osmotic situation but, nevertheless, may alter the availability of water. Thus humpback whales
migrating from polar to equatorial waters to breed do not feed, and, as indicated
by their urine composition, do not drink sea-water (BENTLEY, 1963), depending
on their metabolic tissue reserves for water during these months. Terrestrial birds
making transoceanic or transdesert flights must endure similar privations. It is
doubtful if such hormones have any direct effects on osmoregulation. The thyrotrophic hormones (TSH) and corticotrophic hormones (ACTH) influence the general
integrity, growth and secretion of the thyroid and adrenocortical tissues respectively, the former may influence the osmotic balance of some fishes while the latter
assists the regulation of sodium and potassium metabolism in many groups of ver49
nerve fibres to the neurohypophysis (hypothalamic-neurohypophysial tract) and
pars intermedia, or through blood vessels to the pars distalis. The hypothalamicneurohypophysial tract (which also sends some branches to the median eminence)
originates in amphibians and fishes in the praeoptic nucleus, situated in the anterior
region of the hypothalamus, while in the higher tetrapods (amniotes) these fibres
arise from two areas, the paraventricular and supraoptic nuclei. Blood vessels connect neurons, which converge on the hypothalamus (in the region of the median
eminence of lungfishes and tetrapods), with the pars distalis, These capillaries may
form distinct portal vessels, as in some fishes and most tetrapods, or a less well
defined vascular plexus like in many teleosts, In the fishes the arrangement of the
vascular link varies considerably in different species.
The precise anatomical disposition of the tissues of the pituitary gland varies
in the different vertebrate classes. This is summarized in the now classic illustration
of the late J.D. GREEN (Fig . 2.3). One of the more interesting phyletic developments of the tetrapod neurohypophysis is the appearance of a more distinct
median eminence, and its expansion posteriorly to form the pars nervosa (neural
lobe). WINGSTRAND (1966) has suggested that this may be correlated with adaptation to a terrestrial way of life. It is rather interesting that such a change can also
be seen in the lungfishes, which are often considered to be closely associated with
the evolution of the tetrapods. FOLLETT (1963) summarized information about the
hormonal content of the vertebrate neurohypophysis and found its concentration
(moles/kg body wt) to be greater in those species with a well developed pars nervosa .
b) Secretion of the Pituitary Gland
cx.) Pars Distalis. Eight hormones with distinct actions have so far been found to
originate in the pars distalis. These (see Table 2.2) include the gonadotrophic
hormones (FSH and LH or ICSH), which influence breeding cycles, by promoting
the maturation of the germ cells in the ovaries and testes, as well as the production
of the steroid sex hormones. These hormones are sometimes indirectly associated
with a change in the animals' osmotic circumstances. Thus fish, like the salmon and
lamprey, migrate from the sea into rivers in order to breed while eels move from
rivers back into the sea for the same reason. Periodic breeding migration of birds,
mammals and reptiles does not result in such dramatic changes in the osmotic situation but, nevertheless, may alter the availability of water. Thus humpback whales
migrating from polar to equatorial waters to breed do not feed, and, as indicated
by their urine composition, do not drink sea-water (BENTLEY, 1963), depending
on their metabolic tissue reserves for water during these months. Terrestrial birds
making transoceanic or transdesert flights must endure similar privations. It is
doubtful if such hormones have any direct effects on osmoregulation. The thyrotrophic hormones (TSH) and corticotrophic hormones (ACTH) influence the general
integrity, growth and secretion of the thyroid and adrenocortical tissues respectively, the former may influence the osmotic balance of some fishes while the latter
assists the regulation of sodium and potassium metabolism in many groups of ver49