Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/126

 122 organ of locomotion. This clavicle would give a steady and fixed position to the socket, or glenoid cavity of the scapula, admitting of rotatory motion in the fore leg, analogous to that of the human arm. There is in these circumstances a triple accommodation to the form and habits of the Megatherium; 10. a free rotatory power of the arm was auxiliary to its office, as an instrument to be employed continually in digging food out of the ground; 2°. this act of perpetual digging in search of stationary objects like roots, required but little locomotive power; 3°. the comparatively small support afforded to the weight of the body by the fore leg, was compensated by the extraordinary and colossal strength of the haunches and hind legs. In the Elephant, the great weight of the head and tusks require shortness of neck, and unusual enlargement and strength in the fore legs; hence, the anterior parts of this animal are much stronger and larger than its hinder parts. In the case of the Megatherium, the relative proportions are reversed; the head is comparatively small, the neck is long, and the anterior part of the body but slightly loaded in comparison with its abdominal and posterior regions. In the shoulder blade and collar bone there is great provision to give strength and motion to the fore legs; but this motion is not progressive, nor is the strength calculated merely to support the weight of the body. The humerus, (k) articulates with the scapula by a round head, admitting of free motion in various directions, and is small at its upper and middle part, but at its lower end attains extraordinary breadth, in consequence of an enormous expansion of the crests, which rise from the condyles, to give origin to muscles for the movement of the fore foot and toes. The ulna (l) is extremely broad and powerful at its upper extremity, affording large space for the