. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 230 ANNALS OF THE SOUTH AFRICAN MUSEUM from A to Aj is diagrammatically illustrated, as well as how shortening of the posterior half of the skull in the sequence B—6^—62 compensates for this effect. If A is dropped to A^, the average angle (26°) of CA, DA and EA on the jaw axis BA changes to a value of 23° at A^ on the line BA^. There is thus a con- siderable change in the position of A, but a small change in the average angle of pull on the jaw axis. If B migrates to Bj^ the average angle at A^ beco
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 230 ANNALS OF THE SOUTH AFRICAN MUSEUM from A to Aj is diagrammatically illustrated, as well as how shortening of the posterior half of the skull in the sequence B—6^—62 compensates for this effect. If A is dropped to A^, the average angle (26°) of CA, DA and EA on the jaw axis BA changes to a value of 23° at A^ on the line BA^. There is thus a con- siderable change in the position of A, but a small change in the average angle of pull on the jaw axis. If B migrates to Bj^ the average angle at A^ becomes 25°, while if Bj^ moves to Bg the angle becomes 26°. It is therefore clear that a substantial forward migra- tion of B is necessary to cancel the 3° difference in the angle of pull caused by dropping A to A^. In terms of skull morphology this means that a substantial lowering of the snout produces a small increase in the oblique component of the external adductor musculature, which in turn can be compensated for only by a fairly substantial forward migration of the posterior areas of adductor muscle attachment (Fig. 62). However, if forward migration of the posterior areas of attachment of these adductor muscles involves a forward displacement of the quadrate (as indicated in Figure 62), the jaw axis would undergo a series of new orientations, which could be compensated for only by an increase in the height of the muscle attachment areas. This skull deepening would assist in producing the original angle of pull by the adductors on the mandibular axis, but would considerably increase their length. This lengthening effect is shown in Figure 62, where simple dropping of the snout results in an average increase of 20% in the muscle length. If shortening of the temporal region takes place, the average length of the muscle fibres returns to the original value, but now the distribu- tion of long and short fibres is changed. Thus, in the primary stage of Figure 62, before
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
