SKULL OF A HUMAN FETUS OF 40 MM. 333 



with the lower border of the parietal plate; the lower border 

 forms the lateral and dorsal boundary of the primitive foramen 

 magnum. 



When regarded from the front (fig. 2) the lateral or condyloid 

 portions appear as paired, caudo-lateral extensions of the planum 

 basale, their outer surfaces being simply continuations of the 

 ventral convex surface of the planum; or, stating the same thing 

 another way, if the anterior surface of the planum be regarded 

 as a section of a cylinder, then the outer surfaces of the lateral 

 portions may be looked upon as localized widenings of the same. 

 The upper boundary of each may be arbitrarily marked off by 

 a line drawn from the tip of the occipitocochlear notch to the 

 ventral foraminal prominence (fig. 5), cutting just ventral to the 

 hypoglossal canal, and representing approximately the line of 

 separation which exists between these elements as they occur in 

 their osseous condition at birth. The caudal portion of the 

 planum thus includes the intercondylar incisure. Piercing the 

 outer surface of the lateral portion, which looks ventro-laterally, 

 is seen the outlet of the hypoglossal canal (fig. 2). Upon reach- 

 ing the external edge of the latter the outer surface becomes 

 narrow, and passes directly outward upon the aforementioned 

 processus paracondyloideus. 



Seen from behind, the inner surfaces appear as caudolateral 

 continuations of the side-to-side concavity of the dorsal surface 

 of the planum (figs. 1 and 5). They look medially, dorsally 

 and somewhat cranially, and present the inlet of the hypoglossal 

 canal. The lower borders are by far the thicker, and form the 

 lateral Umits of the incisura intercondyloidea, and each, as has 

 been noted, passes over the ventral foraminal prominence to be 

 thence continued dorsally as the lateral border of the foramen 

 magnum (fig. 2). This portion of the border of the foramen, 

 and the lower border of the condyloid portion, much resemble 

 one another in thickness and roundness on cross-section, and 

 when the skull is regarded from the side the ventral foraminal 

 prominence, formed by their approximation, appears as the 

 apex of an angle directed downwards and slightly forwards (fig. 3) . 



