196 
MASARU NAGAO 
In the second part of the movement, a point (j) on the morsal sur- 
face of the lower central incisors moves upward and forward, starting 
from the position of so-called "edge-to-edge bite" (from jn to jn-^, 
and at the same time the center of the condyle moves forward and 
downward along the articular surface of the tuberculum articulare 
(from Kii to Km), so that the lower jaw as a whole rotates slightly 
around the axis represented by the line connecting the centers of the 
two condyles. This second part of the movement represents a com- 
bined displacement. 
It appears therefore that the antero-posterior movement of the 
lower jaw in the state of sliding contact is a "parallel displacement" 
in the first part, and a "combined displacement" in the second part, 
of the movement; so that when the lower jaw is protruded, starting 
from the resting bite, or is retracted, starting from the position of 
protrusion, it might move either along a curve touching the summits 
of the buccal cusps of the bicuspids and molars (the curve of Spee), 
or along another curve parallel to this. But, in general, the direction 
along which the lower jaw moves is downward and forward in the first 
part and upward and forward in the second part; and thus, although 
the condyle or the points near it move, in the first part, nearly paral- 
lel to the curve of Spee, yet the course of the anterior portion of the 
jaw, or that further away from the condyle, deviates from the curve 
of Spee, the degree of deviation being proportional to the distance 
from the condyle. 
The above relation may be seen clearly from figure 8. The line 
K—Kii along which the condyle moves during the first part of the 
movement is nearly parallel to the curve of Spee, while the line a— 
flii, h-hii etc., along which each cusp moves during the first part, 
diverges from the curve of Spee, the degree of divergence being smaller 
as the condyle is approached. Moreover, the curve of Ki-Ki—Km, 
a—av-avu, bi-bi—bm. etc., or successive positions occupied by the 
moving condyle or cusps during the first and second parts, is neither 
a portion of the curve of Spee nor of a circle homo-centric with it, as 
is shown in figure 8. 
Finally, in the case of man, it is possible in some instances to slide 
the lower jaw antero-posteriorly some distance while in contact with 
the upper jaw. Such a movement of the lower jaw is, however, 
