The longitudinal measurement of the humerus seems to be in direct relation with the length of the metacarpal bones. "In animals supplied with a cannon bone,' it is covered by integument as far as the elbow; it is much elongated, proportionately to the whole body, in orangs, gibbons (long armed apes), bats, and sloths."* In bats, whose anterior extremity is used for striking the air in flight, the articulation of the os humeri with the scapula is a ginglymus or hinge joint, permitting nothing more than the upward and downward movement as seen in birds. No one who has confined his attention to human osteology would believe that the broad flat bone of the upper arm in the mole represents the long cylindrical os humeri in man.
In this animal it articulates not only with the glenoid cavity of the scapula, but also with the clavicle, by an articulating surface, which Cuvier regards as forming part of the greater tuberosity. The crista, from the lesser tuberosity, is so prominent, that it gives to the whole bone an irregular quadrilateral appearance, somewhat twisted, so that the ridge from the lesser tuberosity looks upwards. Of similar form and proportions is the os humeri of the echidna, the terrestrial monotreme of Australia; ' a creature whose powers of rapid burrowing have exeited the wonder of those who have witnessed its disappearance from pursuit by sinking directly downwards into the loose sandy soil over which it was attempting to escape.
* Cuvier, Leons d'Anatomie comparee, vol. i., p. 383.