2 
MacLean 
Figure 2. This section from the brain of a squirrel monkey shows how the greater part of the R-complex is 
selectively colored (black areas) by a stain for cholinesterase (from MacLean, 1972). 
Figure 3 illustrates how the same stain re- 
veals the corresponding structures in animals 
ranging from reptiles to primates. Using the 
histofluorescence technique of Falck and 
Hillarp described in 1959, it is striking to 
see the greater part of the striatal complex 
glow a bright green because of the presence 
of large amounts of dopamine, a neural sap 
that seems necessary for bringing into play 
the whole range of expressive behavior. The 
dopamine is transported to the olfactostria- 
tum and the corpus striatum by nerve proc- 
esses arising from cell bodies in the ventral 
midbrain (ventral tegmental area and com- 
pact cells of substantia nigra)*. Parts of the 
striatal complex are also rich in serotonin 
(Paasonen et al, 1957) and in opiate recep- 
tors (Pert and Snyder, 1973). 
From an evolutionary standpoint, the rep- 
tilian brain is of particular interest because 
it allows one to visualize how developments 
at a critical locus in the so-called hypopallium 
(see Fig. 4) described by Elliot Smith 
(1918-1919) may have tipped the scales so 
* As regards the extensive literature, see particu- 
larly the original articles by A. Dahlstrom and K. 
Fuxe (Acta Physiol. Scand., 232 Suppl., 62: 1-63, 
1964) and J. Ungerstedt (Acta Physiol, Scand., 367 
Suppl., 1-48, 1971). 
