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قراءة كتاب The Adductor Muscles of the Jaw In Some Primitive Reptiles
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The Adductor Muscles of the Jaw In Some Primitive Reptiles
bone.
Insertion: lateral surface of coronoid process of mandible.
3. Internal temporalis
Origin: sagittal crest and skull roof, including posterior two-thirds of frontal bone, whole of parietal, and dorsalmost portions of squamosal and alisphenoid.
Insertion: medial surface of coronoid process; dorsal edge of coronoid process.
Fig. 8. Didelphis marsupialis. Showing masseter and temporal muscles. Skull KU 3780, 1 mi. N Lawrence, Douglas Co., Kansas. × 3/5.
Temporal Openings
In discussions of the morphology and functions of the adductor mechanism of the lower jaw, the problem of accounting for the appearance of temporal openings in the skull is often encountered. Two patterns of explanation have evolved. The first has been the attempt to ascribe to the constant action of the same selective force the openings from their inception in primitive members of a phyletic line to their fullest expression in terminal members. According to this theory, for example, the synapsid opening appeared originally to allow freer expansion of the adductor muscles of the jaw during contraction, and continued selection for that character caused the openings to expand until the ultimately derived therapsid or mammalian condition was achieved.
The second course has been the attempt to explain the appearance of temporal openings in whatever line in which they occurred by the action of the same constant selective force. According to the reasoning of this theory, temporal fenestration in all groups was due to the need to decrease the total weight of the skull, and selection in all those groups where temporal fenestration occurs was to further that end.
Both of these routes of inquiry are inadequate. If modern views of selection are applied to the problem of explaining the appearance of temporal fenestrae, the possibility cannot be ignored that:
1. Selective pressures causing the inception of temporal fenestrae differed from those causing the continued expansion of the fenestrae.
2. The selective pressures both for the inception and continued expansion of the fenestrae differed from group to group.
3. Selection perhaps involved multiple pressures operating concurrently.
4. Because of different genotypes the potential of the temporal region to respond to selective demands varied from group to group.
Fig. 9. Captorhinus. Diagram, showing some hypothetical lines of stress. Approx. × 1.
Fig. 10. Captorhinus. Diagram, showing areas of internal thickening. Approx. × 1.
Fig. 11. Captorhinus. Diagram, showing orientation of sculpture. Approx. × 1.
Secondly, the vectors of mechanical force associated with the temporal region are complex (Fig. 9). Presumably it was toward a more efficient mechanism to withstand these that selection on the cheek region was operating. The simpler and more readily analyzed of these forces are: