Organization of ADVR 
123 
Figure 2. A higher magnification view of a sector 
of ADVR. The lower boundary of cortex is at the 
top of the figure. The ependymal surfaces of cortex 
and ADVR are apposed. Notice that neurons 
throughout ADVR are cytologically similar. How- 
ever, there are differences in their distribution 
which permit the definition of zones A, B, C, and 
D. The approximate boundaries of these zones are 
indicated to the left of the figure. Zone A is cell 
poor and contains a few neurons which are em- 
bedded in the ependyma. Zone B contains clusters 
of touching neurons. Zones C and D contain pri- 
marily isolated neurons. The distinction between 
zones C and D is based on axonal morphology and 
cannot be made in Nissl preparations. 
similar to those of zone B neurons ; however, 
the dendrites bear only a few dendritic 
spines. Their axons extend away from the 
somata in all directions and ramify exten- 
sively, but they never reach more than about 
SOO fi from the somata. The projections of 
neurons in zones A, B, and C, thus, are 
entirely intrinsic to the ADVR. Zone D 
neurons (Fig. 5) lie near the ventral border 
of ADVR. They closely resemble the neurons 
in zone C, except that some zone D neurons 
have axons which course ventrally out of 
ADVR and into the subjacent striatum. 
Zone D neurons are, thus, the only neurons 
in the ADVR known to have extrinsic pro- 
jections. 
EFFERENT PROJECTIONS 
OF ADVR 
It is impossible to trace the axons of zone 
D neurons to their terminations in Golgi 
preparations. Lesions were, therefore, placed 
in the ADVR in a number of snakes and the 
degeneration revealed by a Fink-Heimer 
stain. Selected sections through one of these 
cases are shown in Figure 6. Degeneration 
products circumscribe the lesion involving 
neurons in zones A, B, and C. Degenerated 
axons can be followed ventrally into the 
striatum due to the involvement of zone D 
neurons. Terminal degeneration is localized 
in three striatal structures, called the medial 
striatal nucleus (MS), the intrapeduncular 
nucleus (IP), and the perifascicular com- 
plex (PF). Degeneration products also con- 
tinue caudally into the ventral part of the 
posterior dorsal ventricular ridge (PDVR) 
of Senn and Northcutt (1973). The efferent 
projections of snake ADVR are, thus, con- 
fined entirely to the telencephalon. Any effect 
that ADVR might have on the brainstem 
must be executed through intervening struc- 
tures. 
SYSTEMS AFFERENT TO ADVR 
Two types of axons can be traced from the 
lateral forebrain bundle into the ADVR in 
