Ventral view of the brain: A= cerebrum, B= optic lobe, D= medulla oblongata 1= rostral cerebral pole, 2= caudal cerebral pole, 3= olfactory lobe
, 4= paramedian eminence, 5= ventral longitudinal fissure, 6= olfactory nerve, 7= optic nerve, 8= optic chiasm, 9= optic tracts, 10= infundibulum, 11= hypophysis, 12= oculomotor nerve, 13= trochlear nerve, 14= trigeminal nerve, 15= midbrain 16= pons, 17= ventral medullary fissure.
The olfactory neurons that innervate aesthetascs follow the "aesthetasc pathway"; they terminate on the glomeruli composing the paired olfactory lobes
of the brain (OL).
The connections between the olfactory receptors on the external structures and the olfactory lobes
within the brain are structurally simple and are modest in number of neurons (Sandeman and Sandeman, 1991).
Abbreviations: AL, accessory lobe; OL, olfactory lobe
; ORN, olfactory receptor neuron; LAN, lateral antennular neuropil.
However, it is striking that presumptive neurogenesis of local interneurons of the olfactory deutocerebrum appears to be restricted to species with accessory lobes [ILLUSTRATION FOR FIGURE 7 OMITTED] - that is, glomerular neuropils of the deutocerebrum connected to the olfactory lobes
(Schmidt and Ache, 1997) - whereas neurogenesis of neurons residing in the HBC occurs only in species in which the accessory lobes are thought to be reduced secondarily (Scholtz and Richter, 1995; Sandeman et al., 1993).
Though their brains are smaller than a human's, dogs have 25 times more smell receptors, and their olfactory lobes
are four times bigger.
These sensory neuropils include olfactory lobes
(OLs), which receive input from aesthetasc chemoreceptors (Mellon and Munger, 1990; Sandeman et al., 1992; Schmidt and Ache, 1996b); lateral antennular neuropils (LANs), which are thought to receive input from non-aesthetasc chemoreceptors and mechanoreceptors on both lateral and medial antennular flagella (Schmidt et al., 1992; Schmidt and Ache, 1996a; Roye et al., 2000); and median antennular neuropils (MANs), wh ich receive projections from statocysts, equilibrium receptors, and receptors from the antennular proximal segments (Sandeman et al., 1992; Schmidt et al., 1992; Schmidt and Ache, 1993, 1996a; Cate and Roye, 1997).