130 THE HUMAN SKULL 



most important. If we take a series of skulls and 

 look down upon them from the top or convex 

 surface, from the norma verticalis^ as craniologists 

 call it, the most careless observer will scarcely fail 

 to note that whilst some of these skulls if a suffi- 

 ciently miscellaneous lot has been chosen are 

 nearly as wide as they are long, others are much 

 longer than they are wide. In a word, some heads 

 are literally, not metaphorically long, some 

 broad. To express this relation of length and 

 breadth numerically is the object of what is known 

 as " the horizontal cephalic index," or sometimes, 

 from its paramount importance, " the cephalic 

 index." The length is measured from the glabella, 

 or surface above the root of the nose and between 

 the eyes, to the inion, or external occipital pro- 

 tuberance, a knob of bone at the back of one's head 

 which every person can easily make out. Then the 

 greatest breadth is taken usually just above the 

 ears. The last of these measurements is multiplied 

 by 100 and then the result is divided by the length. 

 The following formula indicates the process : 



Transverse Diameter x 100. 



Antero-Posterior Diameter. 



Those skulls which are much longer than they 

 are broad are called dolichocephalic, the broad 

 skulls are called brachycephalic, and the inter- 

 mediate group mesocephalic. Different writers 

 have assigned different limits for these three groups, 

 but the following table may be taken as fairly 

 representing the present state of opinion on this 

 point : 



