Variability in Position of Cerebral Structures 
To estimate the variation in position of brain structures that one might expect 
in routine experimental procedures, an analysis was made of the coordinates of 
the Commissura anterior, Corpus mammillare and Nucleus habenularis. Anterior 
and posterior limits of these structures for a parasagittal plane were estimated 
from the histological material on 31, 29, and 31 animals, respectively. The 
anterior-posterior extent of each of these structures is plotted in a cumulative 
profile by the solid hnes in figures 3, 4, and 5. Each of these structures typically 
has a maximal extent of 1-1.5 mm.; the rising slope of the solid line curves in 
these figures thas largely represents the anterior limits for each structure while 
the falling slope reflects the posterior boundary. 
The data suggest that there is about a 50% chance of encountering one of 
these structures at its optimal stereotaxic coordinates. The variation between ex- 
tremes for any one point in one of the structures exceeds 4 mm. in AP coordinates; 
however, 90% of the animals show a variation of 1.5 mm. or less from the aver- 
age. The dotted lines represent the curves obtained after displacing all structures 
in any one animal by a constant distance in the anterior-posterior direction. This 
displacement was arbitrarily chosen to bring the coordinates of all of these 
structures for each animal close to the population mean. For different animals, 
the displacement chosen varied from +2.0 to ~1.5 mm. These curves demon- 
strate that there is less variation in the relative position of these structures within 
the brain than that seen for the population. 
14 13 12 II 10 9 
M fs/1 
CORPUS MAMMILLARE 
MM 
NUCLEUS HABENULARIS 
MM 
-yjrei 3 5 — In these fisures the solid lines r^resent distribution profiles of the anterior-posterior 
extent of the Commissura anterior (CoA), Corpus mammillare (M), and the Nucleus habenularis 
fHo in 31, 29, and 31 animals, respectively, as found in routine stereotaxic procedures. 
Dotted lines in these figures represent a replotting of these distribution profiles after adding an 
arbitrary "displacement” to all the coordinate values of each animal (see text). 
i 
