486 0, CHARNOCK BRADLEY, 
and in this way bring about the wedge-like action of the sphenoid, 
with the result that the pterygoids are abducted. In those animals, 
such as the Pseudopus, in which the Mm. pterygo-parietales are at- 
tached to the prootic and not to the parietals, the former part of the 
action is no doubt absent, the action in such cases being simply ab- 
duction of the pterygoids. The constant presence of the M. pterygo- 
sphenoidalis posterior, and the occasional absence of the M. pterygo- 
parietalis, can be explained by saying that the former muscle is 
essential, there being no mechanical provision for raising the frontal 
segment. Whereas the M. pterygo-parietalis is not so essential, since 
the frontal segment tends to descend of its own accord when the 
M. pterygo-sphenoidalis posterior is relaxed. 
The presence of the columella, and its attachment to the prootic 
and pterygoid, will assist in the action of the M. pterygo-parietalis, 
because the upward pressure of the pterygoids will be resisted. 
Since, as we have seen, the M. depressor palpebræ inferioris (WEBER) 
is not entirely inserted into the lower lid, but has also an indirect 
bony insertion, it must be concluded that this muscle also plays a 
part in the production of skull movements. ‘This action will obviously 
be antagonistic to that of the M. pterygo-sphenoidalis posterior, and 
so be in line with that of the M. pterygo-sphenoidalis anterior of the 
snakes. 
In conclusion, it may be well to again revert to*the absence of 
the deep-seated muscles of mastication in the Chameleons, in which 
animals there is no columella, no direct articulation of the quadrate 
and pterygoid bones, and no movement at the upper end of the 
quadrate. 
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