Materials and Methods 
Horsley -Clarke technique 
A Horsley-Clarke apparatus used for the cat or macaque may be adapted 
with little or no modifications for experiments on the squirrel monkey. The hori- 
zontal reference plane of most instruments is based on lines passing through each 
orbitale and the centers of the external auditory meati (approximate Frankfort 
plane ) . As it is undesirable to cut the lower eyelid in chronically prepared ani- 
mals, it is the practice of this laboratory to elevate the level of the eye piece to allow 
for the thickness of the eyelid. A photograph of the eye piece and its alinement 
with the straight ear bars of the Horsley-Clarke apparatus used in the present study 
is shown in figure 1 . The triangular notch in the eye piece is 1 .5 mm. deep and 
3.0 mm. broad; the axial center of the ear bar is on a line with the lower surface 
of the eye piece. The ear bars are inserted directly into the external auditory 
meati; the final distance between tips after insertion usually measures 19-21 mm. 
Figure 1 — Notched eye piece and ear bar of Horsley-Clarke apparatus used in the present study. 
The dotted line connecting center of ear bar and lower surface of eye piece indicates the 
horizontal zero plane. The notch of eye piece allows for thickness of eyelid so that the 
orbitale lies approximately on this plane. 
Animals 
The brain of SM 1 2 for the frontal series depicted in this atlas was chosen for 
illustration because it was most representative of average stereotaxic measurements 
obtained on 31 adult male animals. The mean body weight for these animals 
was 821 g. (standard deviation 127 g., extremes of 550 and 1,050 g. ). 
Procedure 
Multiple marking bars were placed under anesthesia in the brain of each 
animal as a guide to sectioning and to determine the position of frontal and hori- 
zontal planes. In each case X-rays were obtained of the animal’s head to check 
the alinement of bars and to confirm the distance between them. Figure 2 illus- 
trates the lateral skull X-ray of animal SMI 2. 
40 30 20 10 0 -10 -20 
—I 1 1 1 1 1 1- 
40 30 20 10 0 -10 -20 
AP 
Figure 2 — Lateral skull X-ray of monkey SM 12. The brain of this animal was used for making 
the frontal series of this atlas. Marker bars are present bilaterally at AP 18 and AP — 0.5; 
a single horizontal bar falls at H 19.5. The opacity near the ear canal is part of the head 
holder. 
Histological techniques 
At sacrifice all animals were perfused through the heart with 100-200 cc. of 
normal saline at 40° C., followed by an equal amount of a mixture of 10% 
formalin-1. 0% agar agar (U.S.P.) at 40° C. The intact head was placed in 
the refrigerator overnight, and the brain was removed on the following day. The 
2%-3% cm. block to be sectioned was immersed in formalin-agar agar at 40° C. 
for several days; 24 hours before sectioning ethyl alcohol was added to a concen- 
tration of 20%. Serial frozen sections were prepared by the Marshall technique 
( 7 ) with the microtome set at approximately 50 a and cutting in a dorsal to ventral 
direction. The final thickness of sections was calculated by dividing the number 
of sections into the distance between marking bars. Sections were saved at 0.5 
mm. intervals and mounted on glass slides covered with egg albumen. Facing 
sections were respectively stained with cresyl violet and Lillie’s variant of the 
Weil-Weigert method ( 3 ) . To judge distortions inherent in fixation, cutting and 
mounting, dimensions of the sections were compared with measurements taken 
stereotaxically from the base of the animal’s skull and from its skull X-ray. 
Twenty-nine brains were cut in the frontal plane and two in the sagittal plane. 
3 
