172 MAMMALIAN ANATOMY 



The measurements were taken as follows : 



The height of the occipital plate is the distance from the highest 

 point on the superior curved line, at the external occipital protuber- 

 ance, the so-called inion, 1 to the lowest middle point of the thin lower 

 margin of the foramen magnum, the so-called basion. 2 



The width of the occipital plate is its greatest breadth across the 

 jugular processes. 



The height of the part of the external surface above the foramen 

 magnum is the distance from the inion to the middle point of the 

 upper margin of the foramen, a point known as the opisthion. 3 



The height of the foramen magnum is its vertical diameter. The 

 height of the occipital plate is usually less than the height of the 

 part above the foramen added to the height of the foramen itself, 

 because it is measured on the shortest line between the inion and the 

 basion. 



The width of the foramen magnum is taken between the points 

 where the coudyles diverge from the lateral margins ; it represents, 

 therefore, the greatest width and lies above the centre. The spread of 

 the condyles is the greatest distance between their outer edges. 



The length of the basilar process is the distance on its lower 

 surface from the edge of the foramen magnum; that is, from the 

 basion to the middle of the anterior border, a point difficult to locate 

 accurately in skulls wherein the occipital and the sphenoid have 

 completely coalesced. 



The width of the basilar process is the distance between the inner, 

 concave margins of the jugular notches, just in front of the anterior 

 openings of the anterior condyloid canals. 



The relation between two measurements of any part of the skeleton 

 is usually expressed by means of a so-called index. This is derived 

 from a proportion which shows the relation between the measurements 

 when one measurement is put at one hundred. For example, the 

 proportion for the index of height of the occipital plate is : 



Width : height : : 100 : x. Index of height = height -, 



width 



26 X 100 



or -^ = 90.32; or 36 



1 (Gr.) the back of the head. 2 From basis (Gr.), the base. 



3 From opistliios (Gr.), hinder. 



