CURVE OF SPEE IN MAMMALS 
197 
possible only for a short distance, because anteriorly the morsal sur- 
face of the lower central incisors strikes the lingual surface of the 
upper central incisors at once, and posteriorly the head of the condyle 
strikes the processus postglenoidalis of the fossa at once. The maxi- 
mum distance along which the lower jaw can travel is about 1 to 2 mm. 
Therefore, even if we grant that the jaw moves antero-posteriorly 
along the curve of Spec, it is highly improbable, on account of the 
short distance of such a movement, that the antero-posterior move- 
ment of the jaw has any relation to the curve of Spec, and conse- 
quently the existence of the curve of Spee is not associated with any 
greater efficiency in mastication. 
During the course of this investigation it was found that there are 
some cases in which the jaw cannot perform an antero-posterior move- 
ment such as that just described, owing to the form of the teeth or of 
the temporo-mandibular joint; nevertheless, the presence of the curve 
of Spee is evident. For example, Simia satyrus, Hylobates miilleri, 
Macacus cynomolgus, Nasalis larvatus, Semnopithecus femoraHs, 
Macacus nemestrinus, et. al, cannot perform such antero-posterior 
movements owing to the form of the teeth; in other words, the teeth 
of these mammals show ideal occlusion and thus there is no room for 
such antero-posterior movements when the jaws are in contact. 
Furthermore, Dicotyles sp., the peccary, and Didelphys marsupiahs, 
the opossum, have a temporo-mandibular joint which is "ginglymus" 
as already stated, and for this reason they cannot perform such antero- 
posterior movements at all. Nevertheless in all these mammals the 
curve of Spee is unmistakably present. 
Fick (1911) also opposes the conclusion of Spee for a reason similar 
to my own, namely, that the distance within which the lower jaw 
moves antero-posteriorly is too short. Furthermore, he states that 
when the rotation of the lower jaw takes place around the horizontal 
axis represented by the line which connects the middle of each orbit, 
the "coincident sliding (kongruentes Schleifen)" of the total occlusal 
surface of the teeth does not occur, because the projection of the 
teeth forms an arc not only upon the sagittal plane, but also upon 
the horizontal plane, so that when the lower jaw moves antero-poste- 
riorly the occlusal surface of the bicuspids and molars does not slide 
coincidently, but the sagittal aspects of the teeth only sHde by each 
other. 
