CURVE OF SPEE IN MAMMALS 
195 
not move parallel to the lingual surface of the upper central incisors, 
but its path should be a short arc of a circle. However, it seems to 
me that, even granting that in this instance the lower jaw moves on 
a circle, nevertheless the distance moved is but a few millimeters 
Fig. 8. Diagram Illustrating the Direction of the Movement or the Lower Jaw 
IN the First and Second Parts or the Movement of Protrusion, and 
also the Relation of this Movement to the Curve of Spee 
I II III curve, showing the course of movement of each buccal cusp of the 
lower bicuspids and molars, (j): A point on the morsal surface of the lower incisors and 
(K) the middle point of the condyle. To avoid confusion in the drawing, curves are not 
given for each point, but only for j, a, d, g, i, and K. T . . . . transverse axis passing 
through each tuberculum mandibularis. 
so that under these circumstances the path of the center of the con- 
dyle could be regarded as a straight line. My observations on the 
skulls which exhibit ideal occlusion induce me to conclude that all 
points in the lower jaw move downward and forward as a whole, thus 
producing the movement of the "parallel displacement." 
