THE SKULL, 23 
The basilar portion (a2) as seen from the dorsal or ventral 
surface is oblong and flattened. It is broadest at the junction 
of its middle and last thirds, and tapers toward both ends. It 
presents a cranial end and a caudal end, a dorsal, a ventral, 
and two lateral surfaces. Its caudal end, which helps to form 
the ventral boundary of the foramen magnum, is concave from 
side to side. The cranial end is transversely elongate, about 
five times as broad as high, pointed laterally and roughened 
for attachment to the caudal end of the body of the sphenoid, 
which has a corresponding form. The joint is a synchondrosis. 
The dorsal surface is concave from side to side, forming a 
longitudinal groove in which rest the pons and medulla. 
The concavity is more pronounced caudad, where the bone is 
thinner at its middle. 
The ventral surface is marked by three parallel longitudinal 
ridges. One of them is median and expands caudad into a 
smooth triangular elevated area which extends to the foramen 
magnum. The other two ridges run near the lateral edges of 
the bone. Between them and the median ridge the surface is 
smooth and is depressed caudad. Laterad of each lateral ridge 
is a rough triangular surface overlaid in the natural state by the 
medial edge of the tympanic bulla. 
The lateral surfaces are smooth and sharp and abut against 
the petrous portion of the temporal bone. They pass into the 
lateral portions of the occipital caudad. 
The lateral portions (4) of the occipital arise from the 
caudal margin of the basilar portion in the transverse plane; a 
short distance laterad of the line of junction they turn dorsad 
at an angle of nearly ninety degrees. They form the lateral 
boundaries of the foramen magnum and pass dorsally into the 
squamous portion. 
The external surface of each presents an elongated elevated 
spirally curved surface, the occipital condyle (¢), for articula- 
tion with the atlas. A small part of each condyle is formed 
from the basal portion. The two condyles are separated from 
one another ventrally by a narrow notch, and each extends 
(laterad) along the border of the foramen magnum to a point 
slightly dorsad of the transverse diameter of the latter. 
