Posterior Extremities.
The pelvis of the Megatherium (Pl. 5, Fig. 2. p.) is of vast solidity and expanse; and the enormous bones of the ileum (r) are set nearly at right angles to the spine of the back, and at their outer margin, or crest, are more than live feet asunder, very much exceeding the diameter across the haunches of the largest elephant: the crest of the ileum, (s,) is much flattened, as if by the pressure of the armour. This enormous size of the pelvis would be disproportionate and inconvenient to an animal of ordinary stature and functions; but was probably attended with much advantage to the Megatherium, in relation to its habit of standing great part of its time on three legs, whilst the fourth was occupied in digging.
The pelvis being thus, unusually wide and heavy, presents a further deviation from other animals, as to the place and direction of the acetabulum, or socket which articulates with the head of the thigh-bone (u.) This cavity, in other animals, is usually set more or less obliquely outwards, and by this Obliquity facilitates the movement of the hind-leg; but in the Megatherium it is set perpendicularly downwards, over the head of the femur, and is also nearer than usual to the spine; deriving from this position increase of strength for supporting vertical pressure, but attended with a diminished capability of rapid motion.[1]
- ↑ There is also a further peculiarity for the increase of strength in the manner in which that part, which, in most other animals, is an open space, called the ischiatic notch (Pl. 5, Fig. 2 c.) is nearly closed with solid bone by the union of the spines of the ischia with the elongated transverse processes of the sacral vertebrae, (a.)
Further evidence of the enormous size and power in the muscles of the thigh and leg is afforded by the magnitude of the cavity in the sacrum, (Pl. 5. d,) for the passage of the spinal marrow: this cavity being about four inches in diameter, the spinal marrow must have been a foot in circumference. The extraordinary magnitude also of the nerves which proceeded from it to supply the leg, is indicated by the prodigious size of the sacral foramina.
