TOSHIAKI J. HARA
90
nerves and a long olfactory tract, pedunculated ( Carassius and lctalurus),
or long olfactory nerves and a short olfactory tract, sessile (Anguilla and
Esox). In the young goldfish the olfactory bulb is closely located to the
olfactory lobe while an increase in the length of the olfactory tract may
occur during the growth of the fish (Uchihashi, 1953; Schnitzlein, 1964).
Olfactory nerve fibers of the pike ranged from 0.1 to 0.4 p in diameter
and showed no obvious size difference from other vertebrates (Gasser,
1956). The average number of axons in the central portion of the olfactory
nerve was estimated to be 29/pz in burbots (Gemne and DBving, 1969).
The microscopic structure of the olfactory bulb is also essentially
similar in all vertebrates. In fishes the olfactory bulb is poorly differentiated and the lamination is not so distinct as in higher vertebrates. The
dominant feature of the bulb is the synaptic contact between the olfactory nerve fibers and dendrites of bulbar secondary neurons, mitral and
tufted cells. Single mitral cells of fishes often have several dendrites ending in different glomeruli (Allison, 1953). It is significant that the axon
of a receptor cell does not terminate in more than one glomerulus, and
that each glomerulus receives impulses only from a limited group of
several olfactory receptor cells; this is remarkably different from the
mammalian mitral cells in which only a single main dendrite ends in
each glomerulus. Furthermore, coincident with the lower degree of segregation of glomerular transmission, intrabulbar associational systems
such as recurrent collateral also seem to be less elaborately developed
in fishes (Allison, 1953; Fig. 5).
Information from the olfactory bulbs is conveyed through two main
fiber pathways, the lateral and medial olfactory tracts to the basal telencephalic areas. The medial bundle is thicker than the lateral one; both
are subdivided into two small bundles. The two main fiber bundles contain myelinated nerve fibers with diameters less than 6 . 5 ~ ~
but in the
lateral portion of the medial olfactory tract of Lota Zota the majority of
the fibers are smaller than 0.5 p ( DBving and Gemne, 1965). The number
of these fibers larger than 0.5 p is estimated to be about lo4. The ratio
between receptor cells (primary neurons ) and the myelinated fibers in
the tract (secondary neurons) is therefore 1OOO: 1. This is the same order
of the convergence ratio between receptor cells and mitral cells in the
rabbit (Allison and Warwick, 1949). Some fibers in the medial olfactory
tract run directly to the hypothalamus while some cross in the anterior
commissure. Ascending or centrifugal nerve fibers running to the olfactory
bulbs have also been described (Sheldon, 1912). Recently, Westerman
and Wilson (1968) have reported the presence of numerous synapses of
two types, axo-axonal and axo-glial, within the medial olfactory tract of
the carp, Carassius carassius.
90
nerves and a long olfactory tract, pedunculated ( Carassius and lctalurus),
or long olfactory nerves and a short olfactory tract, sessile (Anguilla and
Esox). In the young goldfish the olfactory bulb is closely located to the
olfactory lobe while an increase in the length of the olfactory tract may
occur during the growth of the fish (Uchihashi, 1953; Schnitzlein, 1964).
Olfactory nerve fibers of the pike ranged from 0.1 to 0.4 p in diameter
and showed no obvious size difference from other vertebrates (Gasser,
1956). The average number of axons in the central portion of the olfactory
nerve was estimated to be 29/pz in burbots (Gemne and DBving, 1969).
The microscopic structure of the olfactory bulb is also essentially
similar in all vertebrates. In fishes the olfactory bulb is poorly differentiated and the lamination is not so distinct as in higher vertebrates. The
dominant feature of the bulb is the synaptic contact between the olfactory nerve fibers and dendrites of bulbar secondary neurons, mitral and
tufted cells. Single mitral cells of fishes often have several dendrites ending in different glomeruli (Allison, 1953). It is significant that the axon
of a receptor cell does not terminate in more than one glomerulus, and
that each glomerulus receives impulses only from a limited group of
several olfactory receptor cells; this is remarkably different from the
mammalian mitral cells in which only a single main dendrite ends in
each glomerulus. Furthermore, coincident with the lower degree of segregation of glomerular transmission, intrabulbar associational systems
such as recurrent collateral also seem to be less elaborately developed
in fishes (Allison, 1953; Fig. 5).
Information from the olfactory bulbs is conveyed through two main
fiber pathways, the lateral and medial olfactory tracts to the basal telencephalic areas. The medial bundle is thicker than the lateral one; both
are subdivided into two small bundles. The two main fiber bundles contain myelinated nerve fibers with diameters less than 6 . 5 ~ ~
but in the
lateral portion of the medial olfactory tract of Lota Zota the majority of
the fibers are smaller than 0.5 p ( DBving and Gemne, 1965). The number
of these fibers larger than 0.5 p is estimated to be about lo4. The ratio
between receptor cells (primary neurons ) and the myelinated fibers in
the tract (secondary neurons) is therefore 1OOO: 1. This is the same order
of the convergence ratio between receptor cells and mitral cells in the
rabbit (Allison and Warwick, 1949). Some fibers in the medial olfactory
tract run directly to the hypothalamus while some cross in the anterior
commissure. Ascending or centrifugal nerve fibers running to the olfactory
bulbs have also been described (Sheldon, 1912). Recently, Westerman
and Wilson (1968) have reported the presence of numerous synapses of
two types, axo-axonal and axo-glial, within the medial olfactory tract of
the carp, Carassius carassius.