194 
MASARU NAGAO 
forward portion measures 2 mm. in thickness and covers the tuber- 
culum articulare. 
Thus it is evident that, so far as my own observations go, the state- 
ment that the steeper the path of the condyle of the lower jaw during 
antero-posterior movement, the shorter will be the radius or vice 
versa, cannot be accepted. 
The statement by Spee that the summits of the cusps of the bi- 
cuspids and molars of the lower jaw, and the condyle, will move On 
the same circle or on two homo-centric circles — the cusps on one 
circle, the condyle on the other — during antero-posterior movement, 
will now be discussed from the standpoint of mechanics. For con- 
venience I shall make a brief statement concerning the mechanism of 
the antero-posterior movement of the lower jaw in man. Starting 
from the so-called "resting bite" of the jaw, we analyse the antero- 
posterior movement of the lower jaw into the following two com- 
ponents according to the motion of the incisors: 
1. A downward and forward movement of the inferior central 
incisors (hereafter called the first part of the movement); 
2. An upward and forward movement of the same (hereafter called 
the second part of the movement). In the first part of the move- 
ment, a point (ji, fig. 8) on the morsal surface of the lower central 
incisors moves downward and forward parallel to the Ungual surface 
of the upper central incisors (moving from point 7*1 to j a, fig. 8), be- 
cause, following the protrusion of the jaw in the condition of sliding 
contact, the point (ji) shdes closely on the Ungual surface of the upper 
central incisors. The remainder of the lower jaw, and consequently 
the center of the condyle, will move in the same direction, i.e., down- 
ward and forward parallel to the lingual surface of the upper central 
incisors. This first part of the movement has been called, by some, 
"parallel displacement (paraUele Verschiebung)." 
According to Eltner this first part of th« movement is a "com- 
bined displacement (combinierte Verschiebung)" rather than a paral- 
lel displacement, because during protrusion the condyle rotates around 
the axis connecting the centers of the condyles, and at the same 
time rotates around the axis connecting the centers of the tuber- 
culum articulare on both sides, while the lower jaw as a whole moves 
downward and forward. The condyle as a whole should therefore 
