THE SKULL THE CEANIUM 173 



By this means occipital plates may be classified into broad and 

 narrow plates, according as the index is above or below a given 

 number. 



VARIATIONS IN DEVELOPMENT. 



The occipital plate may be triangular or almost quadrate, and both 

 forms present narrow and broad types. 



On the external surface the occipital protuberance and the lamb- 

 doidal crest may be very greatly developed, and the occipital crest may 

 extend downward almost to the foramen magnum. 



The angle which the plane of the occipital plate forms with the 

 plane of the foramen magnum is a variable one ; it is greater when 

 the tubercles above the foramen are unusually large. 



The shape of the foramen magnum varies from round to trans- 

 versely oval ; it is not always regular or symmetrical. 



The internal surface of the occipital plate exhibits variations in the 

 size and form of the fossa for the vermiform lobe of the cerebellum. 

 Of forty occipitals examined with reference to this variation, two had 

 the fossa median and symmetrical ; in six it was obscure and irregular ; 

 in thirty it was bowed to the right ; and in two only was it bowed 

 to the left. The fossa may be separated by a transverse ridge from 

 the margin of the foramen magnum, or the two features may be 

 continuous. 



The anterior condyloid canal appears to be always present ; it 

 may, however, be double, completely or partially, on one side or on 

 both sides. In one specimen studied, the anterior opening on the left 

 side was high up in the groove for the lateral sinus. 



The posterior condyloid canal is sometimes absent, on one or on 

 both sides ; it is often double, presenting two anterior and two posterior 

 openings ; or it may be only partially divided, presenting two posterior 

 and one anterior or one posterior and two anterior openings. When 

 there are two posterior openings they are generally close together, but 

 the two anterior openings may be widely separated. If the anterior 

 opening be apparently absent from its usual position in the groove for 

 the lateral sinus, or if it be very small, or single, when the canal 

 appears to be double, an accessory opening will be almost always found 

 in the lateral wall of the anterior opening of the anterior condyloid 

 canal. 



An abnormal foramen is sometimes present in the root of the 



