The sternum or breast-bone (Fig. 12, _e_) is developed in proportion to the size of the three pectoral muscles subservient to flight, and is prolonged beneath into a deep keel-like projection. In the cursorial races, such as the Ostriches and the Apteryx, whose wings are not available for flying, the keel is entirely wanting. Whoever considers the position of the hip-joint in the skeleton of a bird, and reflects how far it is necessarily removed behind the centre of gravity when the bird walks with its body in a horizontal position, will at once perceive that the hinder portion of the spinal column, having to support the whole weight of the body under the most disadvantageous circumstances, and at the same time to give attachment to the strong and massive muscles that wield the thigh, must be consolidated and strengthened in every possible manner, and that even the slight degree of movement permitted in the region of the back would here be inadmissible. Most of the hinder vertebræ are, therefore, solidly conjoined into a single piece (Fig. 12, _r_, _s_), sufficiently strong and massive to bear the great strain to which it is continually subjected, leaving only a few of the hindmost pieces (Fig. 12, _d_) free, upon which the feathers of the tail are supported. The fore limb of a bird, although used for the purpose of flight, when stripped of the feathers and quills that, as we have already seen, form the extensive surface of the wing, will be found very much to resemble the human arm in its general arrangement, presenting only such modifications as are required for the performance of its peculiar function. The framework of the shoulder consists of three bones (Fig. 12, _h_, _i_, _k_), named respectively the _scapula_ or _shoulder-blade_, the _clavicle_ or _collar-bone_, and the _coracoid bone_. The scapula (_h_) is a long and comparatively slender piece placed upon the ribs, and embedded in the muscles, to which it gives attachment. The coracoid bone (_k_) is the strongest piece of the shoulder; it supports the wing at one extremity, while at the opposite it is firmly united to the sternum by a broad and massive joint. But the most peculiarly formed part of the shoulder is the _furculum_, or "merry-thought," as it is usually called (_i i_), consisting of the two collar-bones united, so as to form but a single fork-shaped apparatus, the presence of which materially enlarges and strengthens the shoulder, without unnecessarily adding to the weight. It is by the union of the three last-mentioned bones that a place is made for the socket of the shoulder, with which the wing is more immediately connected.