THE SKULL. 23 



The basilar portion (a) 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 (b] 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 (c], 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. 



