In order to render the following account of the structure of a bird's skeleton intelligible to the non-scientific reader, we have delineated that of the Pigeon, and with this figure before us we shall have but little difficulty in indicating those points with which it is essential that the reader of the present volume should be intimately acquainted. (See Fig. 12.) In the back-bone, or vertebral column, we find three principal regions, each of which will merit distinct notice. The _cervical region_, or that portion belonging to the neck (Fig. 12, _b_), is exceedingly variable in its proportionate length, and forms the only flexible portion of the spine; it performs, indeed, the functions of an arm, at the end of which the beak, the chief instrument of prehension, is situated. The number of vertebræ entering into the composition of this part of the skeleton is very variable; in the Swan there are as many as twenty-three, in the Crane nineteen, while in the Sparrow there are only nine. As a general rule, it may be observed that the neck of a bird is never so short as not to be able to reach to every part of the body; in many aquatic species it is remarkably elongated, whether they swim upon the surface by means of webbed or natatory feet, like the Swan, or wade into rivers and marshes, like the Crane or the Heron. Throughout the entire class a very beautiful contrivance is observable in the S-shaped curvature of this region, the joints of the upper vertebræ being so disposed that they will bend forwards, while in the lower part they can only be bent backwards, thus enabling the bird to lengthen or shorten its neck with the utmost facility and gracefulness. But if flexibility is thus admirably provided for in the cervical region, in the thoracic portion of the skeleton which has to support the framework of the wings, and sustain the efforts of the powerful muscles connected with flight, firmness and rigidity become essential requisites, and, accordingly, in the dorsal region, every means has been employed to prevent those movements which in the neck are so advantageously permitted. The vertebræ of this region (Fig. 12, _c_) are therefore so consolidated as to be almost immovable; and, moreover, splints of bone laid along the back materially add to its stability and strength. There are likewise two sets of ribs, one firmly lashed to the back-bone, the other strongly attached to the sides of the sternum; these dorsal and sternal ribs are moreover united to each other by firm connections, and a thorax is thus formed, possessing sufficient mobility to perform the movements connected with respiration, but still affording a strong basis for muscular action; nay, still further to strengthen this part of the skeleton, from the hinder margin of each dorsal rib a broad flat plate of bone (Fig. 12, _f_) is prolonged backwards, so as to overlap the rib next behind, and thus bind the whole together as firmly as possible.